Turnover of Several Glycolytic Enzymes in Rat Liver

Antibodies were used to isolate three glycolytic enzymes from rat liver after single injections of 3H-leucine. From the rates of decay of specific radioactivity, the following apparent half-lives were determined: lactic dehydrogenase, 6.0 days; aldolase, 4.9 days; and glyceraldehyde 3-phosphate dehydrogenase, 5.4 days. Corrections for reutilization of isotope based on the relative turnover rates of total liver protein determined with 3H-leucine and with 14C-guanido arginine reduced these figures by 42%. The turnover rates show no correlation with the thermal stabilities of the enzymes at physiological temperatures because the thermal half-lives for aldolase and lactic dehydrogenase were estimated to be 130 days and 12,000 days at 37°. These results are consistent with the hypothesis that turnover involves a random enzymatic attack of cellular proteins, possibly following engulfment by lysosomes. The concentration of aldolase in rat liver is nearly 10 times that of glyceraldehyde 3-phosphate dehydrogenase. Our results indicate that this is due to a more rapid synthesis, rather than a slower rate of degradation of the former enzyme compared to the latter.