Ornithine decarboxylase from hepatoma cells and a variant cell line in which the enzyme is more stable.

The half-life of ornithine decarboxylase in HMOA cells, a variant cell line derived from hepatoma tissue culture (HTC) cells, was 5 t o 10 h compared to 14 min in the parental cell line. The half-lives of two other rapidly turning over proteins, S-adenosylmethionine decarboxylase and tyrosine aminotransferase, as well as the turnover of total cellular protein, were the same in the two types of cells, suggesting that other proteins did not share the alteration in turnover observed with ornithine decarboxylase. The possible existence of a mutant enzyme was investigated by purifying ornithine decarboxylase 8000-fold from HTC and HMOA cells, the key step being a pyridoxamine-phosphate affinity column. The enzymes from both cell types behaved in an identical manner throughout the purification procedure, were equally thermolabile, and were inhibited in a similar manner by antiserum to ornithine decarboxylase and by ornithine decarboxylase antizyme. The K,,, of the two purified enzymes for L-ornithine were approximately the same (0.04 mM) as were the Kis for putrescine (0.14 mM). Studies employing [14C]difluoromethylornithine, a radioactive irreversible inhibitor that specifically labeled ornithine decarboxylase, showed that the enzyme from both cell types had a subunit M, of 54,000. The amounts of ornithine decarboxylase in the soluble fractions of logarithmically growing HTC and HMOA cells, as determined by binding of the inhibitor, were 3.5 and 11.0 ng/mg protein, respectively. It is concluded that the increased half-life of ornithine decarboxylase in HMO* cells is not due to a more stable form of the enzyme, the alternative possibility being that a specific deactivation system for the enzyme is altered in these cells.