The inhibitory action of L-dihydroxyphenylalanine (L-dopa) in the presence of a monoamine oxidase inhibitor on protein synthesis in vivo and the partial amelioration of this by methionine supplementation.

Male Wistar rats of various age groups were injected daily over a period of 3 weeks with iproniazid (10 micrograms/g body wt.) and L-dihydroxyphenylalanine (L-dopa; 0.1 mg/g body wt.). On the final day 1 h before the termination of the experiment the animals were injected with L-[14C]valine (0.1 microCi/g body wt.). The specific radioactivity of the valine in the proteins of the subcellular fractions of the tissues examined, relative to the time-integrated mean specific radioactivity of this amino acid in the acid-soluble pools of these tissues, was used to assess protein synthesis. The L-dopa/monoamine oxidase-inhibitor treatment was associated with 30--40% inhibition of protein synthesis. Supplementation of the dietary methionine intake by injection of this amino acid markedly diminished the inhibitory action of the L-dopa/monoamine oxidase-inhibitor treatment on protein synthesis in all fractions examined.