Cooperative interactions of tetrahydrofolate with purified pig kidney serine transhydroxymethylase and loss of this cooperativity in L1210 tumors and in tissues of mice bearing these tumors.

Serine transhydroxymethylase (5,10-methylenetetrahydrofolate: glycine hydroxymethyl transferase, EC 2.1.2.1) purified 200-fold from pig kidneys showed cooperative interactions with tetrahydrofolate with a Hill coefficient (n value) of 3.9 and a substrate concentration at 50% of maximum velocity, the S(0.5) value, of 0.5 mM. The enzyme in mouse liver and kidney homogenates also showed cooperative interactions with tetrahydrofolate. However, the enzyme obtained from L1210 solid tumors of mice, and from livers and kidneys of mice inoculated with L1210 cells exhibited hyperbolic saturation kinetics and gave a Michaelis constant, Km, value of 0.5 mM for tetrahydrofolate. The interaction of serine with the enzyme from pig kidney, from tissues of normal or tumor-bearing mice, or from L1210 tumors was hyperbolic with a Km of 0.9 mM. The specific activities of the enzyme in the L1210 tumor and in mouse liver were 10-fold higher than in pig or mouse kidney. There was no significant change in the levels of the enzyme in mouse liver and kidney on inoculation with L1210 cells. These results suggest that a tumor can bring about biochemical changes in tissues that are distal to the tumor.