Circulating insulin levels are routinely measured using Enzyme-linked Immunosorbent Assay (ELISA). However, ELISA is not sensitive enough to measure cerebrospinal fluid (CSF) insulin levels which are 10-100 times lower than those of plasma: yet ELISA results are often reported for insulin analysis in CSF where it serves as a surrogate for brain interstitial fluid insulin levels. We developed a novel LC-MS/MS based selected reaction monitoring (SRM) proteomics assay for quantitative analyses targeting predetermined peptides derived from trypsin digests of insulin; each peptide depends on specific SRM transitions. We determined insulin levels using our novel insulin SRM proteomics method in the matching plasma and CSF samples that were from individuals fasting ≥12 hours and individuals after 2 hours of steady state hyperglycemia by clamp technology (2 hr SS HG). All samples were obtained in individuals with normal fasting glucose and HbA1C levels. Statistically significant differences were observed in both plasma and CSF between fasting and 2 hr SS HG by two tailed and unpaired student t test and 2-way matched ANOVA as shown in Table below. Our results confirmed that peripheral endogenous insulin appears in CSF at least 2 hours after steady state hyperglycemia in subjects with normal glucose tolerance. Our SRM insulin tryptic peptide panel reliably quantified insulin peptide levels in human CSF samples.
Q. Liu: None. M. Zhu: None. C.W. Chia: None. J.F. O’Connell: None. P. Zhang: None. J.M. Egan: None.
National Institute on Aging