Differential regulation of polyamine synthesis and transmethylation reactions in methylthioadenosine phosphorylase deficient mammalian cells.

The consumption of S-adenosylmethionine during polyamine synthesis and transmethylation reactions yields stoichiometric amounts of 5'-deoxy-5'-methylthioadenosine and S-adenosylhomocysteine, respectively. Information concerning the regulation of the two routes of S-adenosylmethionine metabolism in viable cells under changing growth conditions is limited. The present experiments have measured the time-dependent accumulation of 5'-deoxy-5'-methylthioadenosine and L-homocysteine in the medium of four malignant human and murine cell lines deficient in 5'-deoxy-5'-methylthioadenosine phosphorylase (5'-methylthioadenosine: orthophosphate methylthioribosyltransferase). Included in this group were anchorage-independent and anchorage-dependent cells. The enzyme-deficient cells did not detectably cleave 5'-deoxy-5'-methylthioadenosine, and did not appreciably metabolize homocysteine. A comparison of 5'-deoxy-5'-methylthioadenosine and homocysteine excretion therefore provided a noninvasive method for estimating the relative rates of polyamine synthesis and transmethylation. Early after the release of human CEM lymphoblasts from density dependent growth arrest, 5'-deoxy-5'-methylthioadenosine production increased, and exceeded homocysteine synthesis. 5'-Deoxy-5'-methylthioadenosine formation reached a maximum of 0.9 nmol/12 h per 10(6) cells prior to the onset of exponential growth. The kinetics of homocysteine synthesis were different. Homocysteine accumulation was proportional to the specific growth rate, and achieved a peak of 3.1 nmol/12 h per 10(6) cells during mid-exponential phase, at which time 5'-deoxy-5'-methylthioadenosine production was falling. Similar patterns of 5'-deoxy-5'-methylthioadenosine and homocysteine excretion were observed in other 5'-deoxy-5'-methylthioadenosine phosphorylase deficient cell lines. These data show that polyamine synthesis and transmethylation are differentially regulated during the growth cycle of mammalian cells.