Vol. 26, No. 3, 1967 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICAllONS ORIGIN OF PHOTOLABILE MF.THYL GROUPS IN METHIONINE BIOSYNTHESIS* Jonathan D. Brodie Department of Biochemistry Schools of Medicine and Dentistry State University of New York at Buffalo Buffalo, New York 14214 ReceivedDecember 9, 1966 The present experiments were designed to approach the question of whether methyl-B12 is an intermediate in the biosynthesis of methionine from methyl tetrahydrofolate (MeFHb) and homocysteine. In this process, S-adenosyl methionine (SAM) is a required co- factor (Mangum and Scrimgeour, 1962) although its role is still obscure. In addition, methyl-B12 is the coenzyme for methionine synthetase (Weissbach, et al., 1963). For these experiments an enzyme preparation was obtained from a pig liver homogenate and fractionated between 30 and 50% of saturation with ammonium sulfate (Kerwar et al., 1966). This preparation was chosen since this system is independent of the addition of methyl-B12 for enzymatic activity, yet requires SAM for the synthesis of methionine. At this stage of purification there is no demonstrable requirement for an exogenous reducing agent such as reduced flavin (Kerwar et al., 1964). The specific activity of the enzyme as measured by methionine formation (Weissbach et al., 1963) was from 1.6 - 3.0 mp moles of methionine synthesized per hour per mg. of enzyme. The basic approach was to use the known photolability of methyl-BL2 as evidence of the synthesis of this compound. Ac- cording to a recent report, (Hogenkamp, 19661, formaldehyde is a * This investigation was supported by a grant, No. AM10479-01, from the National Institutes of Health and by a U.S.P.H.-General Research Support grant, No. FR-05400-05.