On the metabolism of 3H-tyrosine in the cerebrospinal fluid of the cat: Role of transamination

Abstract

The formation of phenolic end products from tyrosine metabolism was investigated in the cerebrospinal fluid (CSF) of cats after the administration of 3H-tyrosine, 200 μCi intracisternally. Metabolites from dopamine, noradrenaline, tyramine, octopamine and from transamination were searched for at various times, from 10 min to 3 hr, after the 3H-tyrosine injection. Chromatographic evidence is presented for the formation of 3H- p-droxyphenylacetic acid and 3H- p-hydroxyphenyllactic acid. The former acid was the major metabolite and analyses of serial samples of CSF showed that the acid was present in high amounts as early as 10 min after the 3H-tyrosine injection and that considerable radioactivity could be detected also in the 3 hr samples. Relative to the total radioactivity, the highest amounts of 3H- p-hydroxyphenylacetic acid were formed between l.5 and 3 hr after the 3H-tyrosine administration. The labelled phenolic acid was also demonstrated in different brain regions both after intracisternal and systemic administration of 3H-tyrosine. The amounts of 3- p-hydroxyphenylacetic acid formed in CSF were not markedly altered by pretreating the cats with a monoamine oxidase inhibitor (pargyline). It is thought likely that 3H- p-hydroxyphenylacetic acid and 3H- p-hydroxyphenyllactic acid are formed by transamination of 3H-tyrosine through the p-hydroxyphenylpyruvate pathway. The results are discussed in the light of the possible functional relation of transamination to the metabolism of tyrosine as a precursor of catecholamines.