Accuracy of portein synthesis and in vitro aging. Search for altered enzymes in senescent cultured cells from human livers.

The authors have looked for altered proteins in senescent cultured cells from adult liver. Four enzymes (phosphoglycerate kinase, M2 type pyruvate kinase, glucose phosphate isomerase and glucose-6-phosphate dehydrogenase) have been studied by immunological and enzymatic titration and electrofocusing. In addition, heat stability of glucose-6-phosphate dehydrogenase (G6PD) was appraised in cell crude extracts and in partially purified preparations. Enzymatic aactivity as well as immunological reactivity of the four enzymes studied were identical 16 lines in phase II and in 13 lines in phase III. Electrofocusing pattern of the enzymes from 'young cells' was identical to the ones from 'old cells'. Finally, G6PD from old cells seemed to be more unstable than G6PD from young cells when studied in crude extracts. These differences, however, disappeared as G6PD was partially purified from old or young cultured cells. Consequently, no evidence of altered protein, either missynthesized or posttranslationally modified, was found in the senescent cultured cells studied. Moreover, this work indicated that the modification of heat stability of G6PD from old cells was not due to the enzyme molecule itself but rather to the cell medium.