AGE-RELATED-CHANGES IN CARBOXYL METHYLATION OF PROTEINS IN THE KIDNEY

Age-related changes in the carboxyl methylation activities of L-isoasp artyl/D-aspartyl methyltransferase (PIMT) and C-terminal isoprenylcyst eine methyltransferase (PPMT), as well as in the methylation levels of their major substrates, were studied in the soluble and brush border membrane (BBM) fractions of kidney cortex isolated from rats aged 3 we eks, and 2, 7 and 12 months. PIMT activity measured with ovalbumin, an exogenous substrate, decreased by 30% in the soluble fraction, while it increased by 37% in BBM of rats older than 2 months. In the soluble fraction, the affinity of PIMT for the universal methyl donor, S-aden osyl-L-methionine, was unaffected, while the apparent maximal velocity measured with ovalbumin was 30% lower in 7-month-old rats than in 3-w eek-old rats. However, the amount of PIMT measured by Western blotting with anti-PIMT antibodies in the soluble fraction was not affected by age. These results suggest that a reduction in the specific activity of PIMT in the soluble fraction occurs as a function of age. Stimulati on of the methylation of total proteins by guanosine 5'-3-O-(thio) tri phosphate (GTP gamma S) increased in the soluble fraction of rats olde r than 2 months, (30%) and decreased in BBM of rats older than 7 month s (25%). The PIMT methylation of endogenous substrates of 48 and 61 kD a in the soluble fraction decreased by 40% in rats older than 2 months , but no significant difference was found for substrates in the BBM fr action as a function of age. On the other hand, the PPMT activity was stable from 3 weeks postnatal to adulthood. The C-terminal carboxyl me thylation of the major PPMT substrates in BBM (22, 26, and 44 kDa) rem ained stable throughout development and in adults. The levels of carbo xyl methylation of the 22 and 26 kDa substrates in BBM were GTP gamma S-dependent, but only the effect on the 22 kDa substrate was regulated by age. These data suggest that the activities of PIMT and PPMT are r egulated differently during development and aging in the rat kidney co rtex.