Alterations in proline metabolic enzymes with mammalian development.

Through the use of specific radioisotopic assays, the activities of enzymes degrading and synthesizing proline were examined in rat liver and kidney as a function of development. Proline oxidase (PO), the enzyme converting proline to delta 1-pyrroline-5-carboxylate (PC), undergoes 15- and eight-fold increases in liver and kidney, respectively, as rats mature from term-fetal to adult life (6-12 wk). The differences are not due to enzyme inhibitors or activators, and kinetic analysis reveals the change to be one of greater tissue content of the same enzyme. delta 1-Pyrroline-5-carboxylate dehydrogenase, which converts PC to glutamate, shows a two- to three-fold increase in both tissues, paralleling the changes in PO with development. delta 1-Pyrroline-5-carboxylate reductase (PCR), the enzyme which catalyzes the committed step in endogenous proline formation, undergoes oppositely directed changes, such that adult levels are only 20%-25% of fetal levels in liver and kidney. PO/PCR ratios are 25- to 50-fold greater in adult central tissues than they are in fetal tissues. Thus, the central tissues of adult rats appear to function as proline utilizers, whereas those of young rats are chiefly proline formers. This difference may relate to different rates of utilization of proline for protein synthesis in young and adult rats.