Recent animal studies have identified several polypeptides or proteins that can reverse or accelerate aging phenotypes. These candidates include growth/differentiation factor 11 (GDF11), growth/differentiation factor 8 (GDF8), oxytocin, eotaxin, and inhibitors of GDF11 and GDF8: GDF11 and GDF8 propeptides, follistatin, follistatin-related protein 3 (FSTR3), WFIKKN1, and WFIKKN2. These candidate polypeptides or proteins have been difficult to study in the blood using conventional immunoassays, since some of the peptides or proteins exist in multiple isoforms, undergo cleavage or terminal degradation, or have high sequence identity with each other. Whether these proteoforms (defined as the different molecular forms in which the protein product of a single gene can be found, such as isoforms, cleavage, and degradation products) have a similar relationship to aging phenotypes in humans is not known. We will use a novel multiplexed selected reaction monitoring (SRM) assay and liquid chromatography-tandem mass spectrometry (LC-MS/MS) to measure fifteen plasma proteoforms representing eight important proteins in rejuvenation research. We will test the hypothesis that plasma concentrations of these candidate proteoforms change with aging, and, beyond chronological age, predict the development of specific aging phenotypes in adults. The specific aims are: (1) to finalize development of the SRM assay using LC-MS/MS for absolute quantification of plasma proteoforms: GDF11 (propeptide, mature protein), GDF8 (propeptide, mature protein), follistatin (2 isoforms, 1 cleaved form), FSTR3, WFIKKN1, WFIKKN2, eotaxin, oxytocin (nonapeptide, 2 carboxyl-extended forms) and its carrier protein, neurophysin-1, (2) to characterize the relationship of circulating candidate proteoforms with aging phenotypes in the Baltimore Longitudinal Study of Aging and in a replication cohort, the InCHIANTI Study. Aging phenotypes include prevalent and incident sarcopenia, lower extremity physical performance, disability, cognition, memory, cardiac hypertrophy, and insulin resistance. By the end of the project, we should be able to verify or refute the role of these candidate proteoforms in phenotypes of human aging.