Optimal conditions for adenosylmethionine synthesis by yeast adenosylmethionine synthetase are pH 9, 0.1 MK+, and an excess of 0.005 m Mg2+ over that bound as magnesium adenosine triphosphate. At high pH or low ATP, higher concentrations of magnesium are inhibitory but the inhibition is partially overcome by increasing the potassium concentration. Tripolyphosphate hydrolysis is about twice as fast as adenosylmethionine synthesis and its pH vs. activity curve is flat between pH 6 and 8.5. The rate of adenosylmethionine synthesis from selenomethionine and its pH vs. activity curve resemble those of the tripolyphosphatase activity rather than those of adenosylmethionine synthesis from methionine. The kinetic results of adenosylmethionine synthesis and tripolyphosphate hydrolysis including product inhibition patterns and both stimulation and inhibition of the tripolyphosphatase by adenosylmethionine can be rationalized by a branched model with a single binding site for each substrate or product. © 1969, American Chemical Society. All rights reserved.
