An Enzymatic Derivative Double Isotope Assay for l-Methionine

Abstract A highly sensitive and specific enzymatic derivative method for the measurement of l-methionine is described. The procedure depends upon the addition of tracer quantities of l-[methyl-3H]methionine and its conversion to [3H, 14C]-S-adenosyl-l-methionine in the presence of [8-14C]ATP and of purified Escherichia coli ATP:l-methionine S-adenosyltransferase (EC 2.4.2.13). The S-adenosyl-l-methionine is separated from the radioactive reactants and its 14C:3H ratio is a linear function of the quantity of l-methionine originally present. The method has been applied to the measurement of l-methionine in various tissues.