The Synthesis and Degradation of Fructose Diphosphate-Aldolase and Glyceraldehyde 3-Phosphate Dehydrogenase in Rabbit Liver

Fructose diphosphate aldolase and glyceraldehyde 3-phosphate dehydrogenase have been crystallized from rabbit livers after single or sequential injections of [14C]- and [3H]-lysine. The specific radioactivity of the lysine in the dehydrogenase rises continuously and reaches a maximum during the 5th day after injection, whereas that of the aldolase declines exponentially from the middle of the 1st day. There is sufficient information in the concurrent time curves to permit a numerical solution of the simultaneous equations for the time dependence of activity decay of the free lysine precursor pool and the rate constants for the synthesis and degradation of both proteins, internally corrected for the recycling of isotope. The first order rate constants of degradation of aldolase and the dehydrogenase are 0.62 and 0.25 day-1 corresponding to half-lives of 1.1 and 2.8 days, respectively. Following an initial rapid decline, the specific activity of the effective free lysine pools tends to plateau at a level intermediate between those of the lysines in aldolase and the dehydrogenase and becomes equal to that of the dehydrogenase when the latter reaches its maximum. During this period, the radioactivity of the free lysine is replenished by the breakdown of the most rapidly metabolized fraction of liver proteins. As measured by the uncorrected raw data, the apparent half-life of lysine-labeled aldolase is 3.86 days and of leucine-labeled aldolase 1.48 days. Leucine is less extensively recycled. In contrast with efforts to obtain true pulse labeling conditions by using a rapidly cleared isotope, the present analysis is facilitated by the use of lysine, an amino acid that is most extensively reutilized.