METHOD FOR DETERMINATION OF HYPERPHOSPHORYLATED TAU IN HUMAN CEREBROSPINAL FLUID BY LC-MS

Abstract

The present invention relates to a method of measuring pS396 Tau in a sample comprising the steps of i-iv, i) Treating a CSF sample from a subject suffering or suspected to be suffering from a Tau pathology with trypsin, ii) Subjecting the sample in step i) to a desphosphorylation agent and subsequently take part of the sample aside for step iv) before proceeding to step iii) with the remaining sample, iii) Subjecting the dephosphorylated sample from step ii) to a second trypsin treatment, iv) Measuring the amount of the Tau peptides (SPVVSGDTSPR) corresponding to Tau residues 396-406 in the samples from step ii) and step iii) using LC-MS.

Claims

1. A method of measuring pS396 Tau in a sample comprising the steps of i-iv, i) Treating a CSF sample from a subject suffering or suspected to be suffering from a Tau pathology with trypsin, ii) Subjecting the sample in step i) to a desphosphorylation agent and subsequently take part of the sample aside for step iv) before proceeding to step iii) with the remaining sample, iii) Subjecting the dephosphorylated sample from step ii) to a second trypsin treatment, iv) Measuring the amount of the Tau peptides (SPVVSGDTSPR) corresponding to Tau residues 396-406 in the samples from step ii) and step iii) using LC-MS.

2. The method according to claim 1, further comprising a step of comparing the amount of the Tau peptides (SPVVSGDTSPR) from step ii) and step iii) as measured in step iv).

3. The method according to claim 1 or 2, further comprising a step of comparing the results obtained in step iv) with a control comprising the Tau residues 260-267 (IGSTENLK).

4. The method according to claims 1-3, wherein the subject is human.

5. The method according to any one of the previous claims, wherein the Tau pathology is selected from Alezheimer's Disease, Down's Syndrome, Argyrophilic Grain Disease (AGD), Psychosis, particularly Psychosis due to AD or Psychosis in patients with AD, apathy due to AD or apathy in patients with AD, psychiatric symptoms of patients with Lewy body dementia, Progressive Supranuclear Palsy (PSP), Frontotemporal dementia (FTD or variants thereof), TBI (traumatic brain injury, acute or chronic), Corticobasal Degeneration (CBD), Picks Disease, Primary age-related Tauopathy (PART), Neurofibrillary tangle-predominant senile dementia, Dementia pugilistica, Chronic traumatic encephalopathy, stroke, stroke recovery, neurodegeneration in relation to Parkinson's disease, Parkinsonism linked to chromosome, Lytico-Bodig disease (Parkinson-dementia complex of Guam), Ganglioglioma and gangliocytoma, Meningioangiomatosis, Postencephalitic parkinsonism, Subacute sclerosing panencephalitis, Huntington's disease, lead encephalopathy, tuberous sclerosis, Hallervorden-Spatz disease and lipofuscinosis.

6. The method according to any one of the previous claims wherein the Tau pathology is Alzheimer's Disease or Down's Syndrome.

7. Use of the method according any one of the previous claims for diagnosing a patient with a Tau pathology.

8. Use of the method according any one of the previous claims for monitoring the disease of a patient with a Tau pathology.

9. Use of the method according any one of the previous claims for monitoring a treatment effect in the subject suffering from a Tau pathology.

10. The use according to claim 9, wherein the treatment is an anti-Tau antibody treatment.

11. The use according to any one of claims 7-10, wherein the Tau pahthology is selected from Alzheimer's Disease, Down's Syndrome, Argyrophilic Grain Disease (AGD), Psychosis, particularly Psychosis due to AD or Psychosis in patients with AD, apathy due to AD or apathy in patients with AD, psychiatric symptoms of patients with Lewy body dementia, Progressive Supranuclear Palsy (PSP), Frontotemporal dementia (FTD or variants thereof), TBI (traumatic brain injury, acute or chronic), Corticobasal Degeneration (CBD), Picks Disease, Primary age-related Tauopathy (PART), Neurofibrillary tangle-predominant senile dementia, Dementia pugilistica, Chronic traumatic encephalopathy, stroke, stroke recovery, neurodegeneration in relation to Parkinson's disease, Parkinsonism linked to chromosome, Lytico-Bodig disease (Parkinson-dementia complex of Guam), Ganglioglioma and gangliocytoma, Meningioangiomatosis, Postencephalitic parkinsonism, Subacute sclerosing panencephalitis, Huntington's disease, lead encephalopathy, tuberous sclerosis, Hallervorden-Spatz disease and lipofuscinosis.

12. The use according to any one of claims 7-10 wherein the Tau pathology is Alzheimer's Disease or Down's Syndrome.

Description

DESCRIPTION OF THE FIGURES

[0017] FIG. 1. Schematic diagram of the full-length Tau protein and the selected peptides, Tau(260-267) and Tau(396-406) for LC-MS analysis. The figure also indicates the position of phosphorylation on S396 (pS396) and the peptide TDHGAEIVYK[p]SPVVSGDTSPR (Tau(386-406)), which is the result of miscleavage at Lysine 395. pS396-Tau is indirectly measured as the difference in Tau peptide (amino acids 396-406) concentration before and after dephosphorylation.

[0018] FIG. 2. Theoretical LC-MS/MS chromatograms showing the signature peptides obtained in the individual steps of the sample workflow for analysis of S396 phosphorylated Tau, exemplified by 30% phosphorylation on S396. SPVVSGDTSPR (SEQ ID NO: 1) is measured in the method, whereas TDHGAEIVYKSPVVSGDTSPR (SEQ ID NO: 2) is only included in this figure for reference.

[0019] Top panes (A+B) show the analytical peaks after first tryptic digest and dephosphorylation: A: The analytical peak for TDHGAEIVYKSPVVSGDTSPR correspond to Tau phosphorylated on S396 (miscleavage) whereas B: SPVVSGDTSPR correspond to Tau not phosphorylated on S396.

[0020] The bottom panes (C+D) show analytical peaks obtained after second tryptic digest: C: TDHGAEIVYKSPVVSGDTSPR is not detectable as this is now cleaved at the Lysine 395 site (S396 is no longer phosphorylated). D: The analytical peak for SPVVSGDTSPR increases with an area corresponding the peak in pane A. This additional area corresponds to the amount of Tau phosphorylated at S396 (pS396).

[0021] FIG. 3A-B. LC-MS/MS chromatograms for analysis of endogenous levels of Tau(396-406) SPVVSGDTSPR peptide in human CSF. Panel A show the chromatogram obtained after workflow 1 (i.e. after first tryptic digest and de phosphorylation) and Panel B show the chromatogram after workflow 2 (i.e. after second tryptic digest). Note that the analytical peak area increases from 30489 counts in WF1 to 52585 counts in WF2, equivalent to a difference of 22096 counts or 42%. This difference corresponds to the amount of phosphorylation on S396 (pS396).

[0022] FIG. 4A-D. LC-MS/MS chromatograms for analysis of Tau(396-406) SPVVSGDTSPR peptide in calibration standards prepared in artificial CSF. Panel 1 shows a blank sample, Panel 2 show a 2 pM calibration standard corresponding to the lower limit of quantification (LLOQ), Panel 3 show a 20 pM calibration standard (typical level in human CSF) and Panel 4 show a 100 pM calibration standard corresponding to upper level of quantification (ULOQ). In each panel, the Figure a is the non-labelled peptide and Figure b is the internal standard (15N-SPVVSGDTSPR).

[0023] FIG. 5. Data obtained for analysis of CSF samples from transgenic mice (rtg4510) after 6 weeks administration (once weekly) of either a control hIgG antibody (B12, not expected to bind to Tau) or a monoclonal hIgG anti-Tau antibody developed to target the pS396-Tau epitope. Young mice were 3.5 months when CSF were sampled and are known to have low levels of Tau pathology. Old mice were 9 months when CSF were sampled and are known to have severe levels of Tau pathology at that age. Top pane show results for analysis of Tau(396-406) and the bottom pane show results for S396 phosphorylated Tau calculated as the difference between Tau(396-406) before and after the second tryptic digest.

[0024] In the control antibody treated mice, it can be observed that the measured concentrations of tau(396-406) and pS396 is higher in the old compared to young mice. Furthermore, in the old mice (with Tau pathology) a dose dependent reduction of tau(396-406) and pS396 is observed after treatment with anti-Tau hIgG.

SEQUENCES INCORPORATED BY REFERENCE

[0025] SEQ ID NO:1 SPVVSGDTSPR (Tau 396-406) [0026] SEQ ID NO: 2 TDHGAEIVYKSPVVSGDTSPR (Tau 386-406) [0027] SEQ ID NO: 3 IGSTENLK (Tau 260-267)

DESCRIPTION OF THE INVENTION

[0028] The present invention relates to an assay measuring the amount of Tau phosphorylated at the Serine residue S396 (pS396) in a human cerebrospinal fluid (CSF) sample. The aim is to get a measure of hyperphosphorylated Tau in the CSF of subjects by measuring pS396, particular in subjects where a diagnosis is desired or where the effect of a treatment is to be determined or monitored. These subjects are suffering or suspected to be suffering from a Tau pathology such as Alzheimer's disease (AD) and Down's Syndrome.

[0029] The inventors of the present invention have used analysis of pS396 Tau as an indirect measure of hyperphosphorylated Tau in CSF. By using a tryptic cleavage step, the inventors of the present invention have taken the advantage of trypsins inability to cleave Tau just before the Serine residue 396 (S396) in Tau when phosphorylated.

[0030] An aspect of the invention therefore lies in the tryptic digestion of the sample obtained from CSF. If Tau is unphosphorylated at the position S396 trypsin can cleave just before residue S396 and generate a peptide SPVVSGDTSPR [amino acids 396-406]. This fragment is dominant in e.g. healthy subjects without Tau pathology. During the disease stages of a Tau pathology (e.g. AD), Tau will become hyperphosphorylated and the residue S396 will also be phosphorylated. Phosphorylation at S396, generating pS396, will block trypsin's ability to cleave, and tryptic digestion will therefore not give the signature peptide SPVVSGDTSPR. Instead, it will yield a larger peptide TDHGAEIVYKSPVVSGDTSPR.

[0031] In one aspect, the invention relates to a method comprising the steps of [0032] i) Treating a CSF sample from a subject (e.g. a human) with trypsin, [0033] ii) Subjecting the remaining sample in step i) to a desphosphorylation agent (such as a phosphatase) and subsequently take part of the sample aside for step iv) before proceeding to step iii), [0034] iii) Subjecting the dephosphorylated sample from step ii) to trypsin treatment, [0035] iv) Measuring the amount of the peptides SPVVSGDTSPR in the samples from step ii) and step iii) (e.g. using LC-MS), [0036] v) Optionally, comparing the amount of the Tau peptides (SPVVSGDTSPR) from step ii) and step iii) as measured in step iv) [0037] vi) Optionally, comparing the amount of the Tau peptides (SPVVSGDTSPR) from step ii) and step iii) as measured in step iv) with a control, such as a trypsin digested Tau peptide fragment not affected by phosphorylation (e.g. the Tau peptide comprising the residues 260-267 (IGSTENLK)) (SEQ ID NO: 3).

[0038] The measured SPVVSGDTSPR in step ii) above will measure the portion of SPVVSGDTSPR corresponding to unphosphorylated S396 in the CSF sample, because trypsin is only able to cleave between amino acids Lysine 395 and Serine 396 if Serine 396 is unphosphorylated.

[0039] In step iii), a second tryptic digest is applied after the dephosphorylation step in step ii). The portion of phoshorylated and uncleaved Tau peptide TDHGAEIVYKSPVVSGDTSPR from step i) will now be cleaved between amino acids Lysine 395 and Serine 396 since Tau is no longer phosphorylated at Serine 396. Hence the second tryptic digest yields additional amounts of SPVVSGDTSPR corresponding to the sum of unphosphorylated S396 and phosphorylated S396 (total amount of Tau).

[0040] Further, the difference of the amount of SPVVSGDTSPR measured directly after step i) (unphosphorylated S396) and the amount of SPVVSGDTSPR measured after the second digestion in step iii) (total Tau) is the amount of Tau protein phosphorylated at S396. IGSTENLK [260-267] serves as a reference peptide since this peptide does not contain any phosphorylation sites, and the same concentrations should be measured before and after the second tryptic digest step. This serves as a confirmatory peptide for measurement of the total concentration of Tau protein.

[0041] The trypsin digested peptides in step i) are subjected to a solid phase extraction (SPE) prior to dephoshorylation treatment in order to remove residues of trypsin prior to the dephosphorylation step (to avoid cleavage at S396 when phosphorylation is removed). Prior to dephosphorylation, pH is adjusted to 5.5. At this pH the enzymatic activity of trypsin is minimized whereas the activity of Lambda Protein Phosphatase (?PP) is maintained. In step v) a suitable method for analysis of Tau protein in the above-described assay can be an LC-MS analysis of peptides, Tau(396-406) and Tau(260-267).

[0042] The method according to the invention can be used to assess or monitor the Tau pathology of a subject such as a mammal (mouse or monkey) and in particular a human.

[0043] The Tau pathology can be seen in many diseases such as Alezheimer's Disease, Down's Syndrome, Argyrophilic Grain Disease (AGD), Psychosis, particularly Psychosis due to AD or Psychosis in patients with AD, apathy due to AD or apathy in patients with AD, psychiatric symptoms of patients with Lewy body dementia, Progressive Supranuclear Palsy (PSP), Frontotemporal dementia (FTD or variants thereof), TBI (traumatic brain injury, acute or chronic), Corticobasal Degeneration (CBD), Picks Disease, Primary age-related Tauopathy (PART), Neurofibrillary tangle-predominant senile dementia, Dementia pugilistica, Chronic traumatic encephalopathy, stroke, stroke recovery, neurodegeneration in relation to Parkinson's disease, Parkinsonism linked to chromosome, Lytico-Bodig disease (Parkinson-dementia complex of Guam), Ganglioglioma and gangliocytoma, Meningioangiomatosis, Postencephalitic parkinsonism, Subacute sclerosing panencephalitis, Huntington's disease, lead encephalopathy, tuberous sclerosis, Hallervorden-Spatz disease and lipofuscinosis. It is envisaged that the method disclosed above, can be used for diagnosing a patient with a Tau pathology, such as Alzheimer's or Down's syndrome. Further, it is believed that the method will also be useful for monitoring the disease of a subject with a Tau pathology.

[0044] It is further envisaged that the method disclosed above, can be used for monitoring the treatment effect in a subject with a Tau pathology, such as Alzheimer's or Down's syndrome. In particular, it is believed that the method is useful to monitor any treatments that may be targeting the Tau pathology such as anti-Tau antibody treatment.

EXAMPLES

Example 1: Materials and Reagents

[0045] Lambda Protein Phosphatase (?PP, 50 ?L of 400 000 units/mL) and 10 mM Manganese(II)chloride (1 mL of 10 mM MnCl2) was obtained from Bioke.

[0046] Sep-Pak? tC18 100 mg 96-well SPE plate was obtained from Waters, US. Phosphatase inhibitor cocktail Ill was obtained from Sigma Aldrich.

[0047] Trypsin (from porcine pancreas, T0303) was obtained from Sigma Aldrich Trypsin inhibitor was obtained from Sigma Aldrich

Biological Samples

Human CSF obtained from BioIVT or PrecisionMed

[0048] Mouse CSF obtained from own laboratory. CSF sampled from rtg4510 mice expressing human ON4R tau harbouring a disease associated mutation P301L.

Preparation of Reagents

[0049]

TABLE-US-00001 Solution Preparation Mobile phase A 0.1% formic acid:0.5% DMSO in water Mix 2.00 mL formic acid and 10 mL DSMO with 2000 mL water Mobile phase B Methanol Rinse solvent Acetonitrile:2-propanol:water:formic acid (20:40:40:1, v/v/v/v) Mix 200 mL acetonitrile, 400 mL 2-propanol, 400 mL water and 10.0 mL formic acid Seal wash 2-propanol:water (10:90, v/v) Mix 100 mL 2-propanol with 900 mL water 0.1% Tween20 0.1% Tween20 in PBS in PBS Mix 400 ?L Tween20 with 40 mL PBS(10x) and 360 mL water Store at room temperature Expiry date: One week after preparation Artificial CSF 0.1% Tween20 in PBS + phosphatase inhibitor (aCSF) Mix 1 mL phosphatase inhibitor cocktail III with 50.0 ?L Tween20 with 5.00 mL PBS(10x) and 45.0 mL water Store as blank matrix together with the samples ?70? C. Digestion solvent 250 mM ABC in water. Dissolve 4.94 g ammonium bicarbonate in 250 mL water Store at room temperature Expiry date: One week after preparation Dephosphorylation 100 mM Ammonium acetate pH = 5.5. solvent/SPE wash Dissolve 7.71 g ammonium acetate in 1000 mL solvent water, adjust to pH 5.5 with acetic acid. Store at room temperature Expiry date: One week after preparation 1 mM HCl in water Mix 83.3 ?L HCl (37%) with 1000 mL water. Store at room temperature Expiry date: 3 months after preparation Trypsin solution 5 mg/mL Trypsin in 1 mM HCl in water Weigh 200 mg trypsin (T0303) and dissolve in 40.0 mL 1 mM HCl in water. If the trypsin weight is more or less than 200 mg, adjust the added volume of 1 mM HCl in water. Aliquot volume: 1.00 mL in 1.5 mL PP cups. Store until use at ?20? C. Set the expiration date 1 year after preparation. Aliquots are single use only!!! 1% formic acid 1% formic acid in water in water Mix 500 ?L formic acid with 50 mL water Store at room temperature Expiry date: One week after preparation
Following Reagents are Prepared Freshly During Sample Preparation. Details of Preparation are Described Under Sample Preparation: [0050] Digestion mix 1 100 ?g/mL trypsin in 250 mM ABC in water [0051] Trypsin stop solution 1.00 mg/mL trypsin inhibitor in water [0052] Manganese(II)chloride solution 3.125 mM MnCl2 in water [0053] Lambda protein phosphatase (?PP) solution 36364 units/mL ?PP in water [0054] Digestion mix 2 300 ?g/mL trypsin in 250 mM ABC in water

Stock Solutions

[0055] Stock solution .sup.15N-Tau protein (1.00 mg/mL) (=21.6 ?M, MW=46353.2 g/mol)Dissolve an equivalent of 100 ?g in 100 ?L 0.1% Tween20 in PBS

Stock Solution pS396 Tau Protein (?1.20 mg/mL) (=26.1 ?M, MW=45902.2 g/Mol)

[0056] This solution is used to demonstrate method suitability and determine the lowest percentage of S396-phosphorylation that reliable can be measured

Example 2: Procedure for Sample Preparation-Workflow 1

First Tryptic Digest

[0057] Frozen CSF samples were thawed at room temperature and homogenized by slowly inverting (at least 5 times) and vortexing for 30 seconds. Samples are centrifuged for 1 minute at approximately 500? g and 20? C. and 200 ?L sample is pipetted into a 1 mL plate. Add 10.0 ?L 0.1% Tween 20 in PBS to the (double) blanks and add 10.0 ?L internal standard working solution (2.7 nM .sup.15N-Tau protein) to all other wells.

[0058] Prepare the digestion mix 1 freshly as described under materials and reagents (100 ?g/mL trypsin). Mix 100 ?L 5 mg/mL trypsin in 1 mM HCl solution with 4900 UL digestion solvent (250 mM ABC in water) in a tube. Add 40.0 ?L of the freshly prepared digestion to each well and vortex the plate for 60 minutes at 1200 rpm (thermomixer) and 37? C.

[0059] Prepare the digestion stop solution freshly (1.00 mg/mL). Weigh an amount of approximately 1.5 mg trypsin inhibitor and dissolve in the required volume of Milli-Q to achieve a concentration of 1.00 mg/mL.

[0060] Add 12.0 ?L of the freshly prepared digestion stop solution to each well and vortex the plate for 2 minutes at 1200 rpm (thermomixer or mixmate) and 37? C.

[0061] Add 60.0 ?L of digestion solvent (250 mM ABC in water) and 300 ?L of 0.1% Tween 20 in PBS to each well and vortex-mix.

Solid-Phase Extraction

[0062] A solid-phase extraction step is applied to remove residues of trypsin prior the dephosphorylation step (to avoid cleavage at S396 when phosphorylation is removed) and the concentrate the sample.

[0063] Condition the SPE-columns (Sep-Pak? tC18 100 mg) with 500 ?L methanol followed by condition with 1000 ?L Milli-Q water. Load the complete sample onto the SPE-columns. Wash the SPE-columns twice with 1000 ?L SPE wash (100 mM ammonium acetate pH=5.5).

[0064] Place the SPE plate on a (clean) 500 ?L plate and centrifuge the SPE plate on the 500 ?L plate for 1 minute at 400*g to collect all the remaining SPE wash (waste). Elute the samples with 450 UL methanol into a new clean 500 ?L LoBind plate deep well plate.

[0065] Evaporate the solvent to dryness at 65? C. under a gentle stream of nitrogen for approximately 45 minutes and reconstitute the samples with 20.0 L dephosphorylation solvent (100 mM Ammonium acetate pH=5.5). At this pH the enzymatic activity of trypsin is minimized whereas the activity of Lambda Protein Phosphatase (?PP) is maintained.

Dephosphorylation Step

[0066] Prepare the MnCl2 3.125 mM solution by mixing 1000 ?L of 10 mM MnCl.sub.2 with 2200 UL Milli-Q in a tube. Add 20.0 ?L of the (freshly) prepared 3.125 mM MnCl.sub.2 solution to the samples. Prepare the ?PP solution fresh, by mixing 50.0 ?L ?PP solution (containing 20 000 units) with 500 ?L Milli-Q in the original cup. Add 10.0 ?L of the (freshly) prepared ?PP solution to the samples and vortex the plate for 120 minutes at 1200 rpm (thermomixer) and 37? C.

[0067] Add 50.0 ?L dephosphorylation solvent (100 mM Ammonium acetate pH=5.5) to the plate and vortex the plate for 1 minute at 1200 rpm (thermomixer) and 37? C.

Finalize Workflow 1 and Prepare for Workflow 2

[0068] Transfer 50.0 ?L of the sample to a new 500 ?L protein plate. This sample will be used in workflow 2 and undergo a second tryptic digest (workflow 2 plate).

[0069] To the original plate, add 50.0 ?L 1% formic acid in water and vortex the plate for 1 minute at 1200 rpm and RT. This plate is ready for LC-MS/MS analysis.

Second Tryptic Digest

[0070] Prepare the digestion mix 2 freshly (300 ?g/mL trypsin) by mixing 300 ?L 5 mg/mL trypsin in 1 mM HCl solution with 4700 ?L 250 mM ABC solution in a tube. Add 30.0 ?L of the freshly prepared digestion mix 2 to each well of the workflow 2 plate and vortex for 60 minutes at 1200 rpm (thermomixer) and 37? C.

[0071] Stop the digestion by adding 20.0 ?L 1% formic acid in water to the workflow 2 plate and vortex the plate for 1 minute at 1200 rpm and 37? C. This plate is now ready for LC-MS/MS analysis.

Example 3: LC-MS/MS Analysis

[0072] Separation from interfering endogenous compounds is achieved by LC-MS/MS using an Acquity HSS T3 (100?2.1 mm, 1.8 ?m) analytical column, set at 30? C., and 0.1% formic acid and 0.5% DMSO in water as mobile phase A and methanol as mobile phase B. An 11-minute gradient at a flow rate of 0.500 mL/min is applied running at 5% mobile phase B for the first 1 minute, linearly increasing to 14% B after 9 min, followed by a step to 90% B which is maintained for 1 min before decreasing back to 5% B. Injection volume is 40 ?L.

[0073] The expected retention time for SPVVSGDTSPR [396-406] is 7.2 min and for IGSTENLK [260-267] it is 6.5 min.

[0074] A SCIEX triple quad 6500 mass spectrometer equipped with a turbo ion spray source is used for detection in positive ion mode. The turbo ionspray source is operated in the positive ion mode at an ion spray voltage of 5500V and at a temperature of 500? C. Curtain gas is set at 30.

[0075] Quantification is based on multiple reaction monitoring (MRM) using the transitions specified in Table below. A linear (Analyst) calibration curve with a 1/x2 weighting factor is used ranging from 2 to 100 pM Total Tau protein in CSF.

TABLE-US-00002 Mass dependent parameters Q1 Mass Q3 Mass Time DP CE CXP (Da) (Da) (msec) (V) (V) (V) IGSTENLK 431.28 748.5 75 50 20 20 [260-267] 431.29 748.5 75 50 20 20 5 MRM transitions 431.30 748.5 75 50 20 20 are summated 431.31 748.5 75 50 20 20 431.32 748.5 75 50 20 20 .sub.15N-IGSTENLK 436.30 757.5 100 50 20 20 [260-267] SPVVSGDTSPR 551.28 719.30 75 70 27 15 [396-406] 551.29 719.30 75 70 27 15 5 MRM transitions 551.30 719.30 75 70 27 15 are summated 551.31 719.30 75 70 27 15 551.32 719.30 75 70 27 15 .sub.15N-SPVVSGDTSPR 558.30 729.30 100 70 27 15 [396-406]