Long-lived metabolic enzymes in the crystalline lens identified by pulse-labeling of mice and mass spectrometry

The lenticular fiber cells are comprised of extremely long-lived proteins while still maintaining an active biochemical state. Dysregulation of these activities has been implicated in age-related cataracts, and other lens diseases. However, the lenticular protein dynamics underlying health and disease is unclear. We sought to measure the global protein turnover rates in the eye using dietary nitrogen-15 (15N)-labeling of mice between 3 and 15 weeks of age. By performing mass spectrometry we measured the 14N- to 15N-peptide ratios of 248 lens proteins, including Crystallin, Aquaporin, Collagen and Laminin of the lens capsule, and enzymes that catalyze glycolysis as well as oxidation and reduction reactions. Unexpectedly, like the crystallin proteins, many of these enzymes are also exceedingly long-lived. The slow replacement of these enzymes in spite of young age of the mice suggests their potential roles in age-related metabolic changes in the lens.