Aromatic l-amino acid decarboxylase activity in central and peripheral tissues and serum of rats with l-dopa and l-5-hydroxytryptophan as substrates

Abstract

This paper describes the distribution of aromatic l-amino acid decarboxylase (AADC) activities in seventeen tissues (eleven peripheral and six brain tissues) of rats, using both l-DOPA and l-5-hydroxytryptophan (l-5-HTP) as substrates. The ratios of the activities of the enzyme using both substrates in the same homogenates were also determined. Rat pineal gland had the highest activity with the substrates followed by the liver, kidney and adrenals. Activity in the adrenals was not only high, but the ratio of the activities for l-DOPA and l-5-HTP was the highest. AADC activity was detected in rat serum by a new and highly sensitive high-performance liquid chromatography-voltammetric assay, using l-DOPA and l-5-HTP as substrates; the ratio of the activities for the two substrates was the lowest. Km values, measured for the two substrates using the homogenates of the pineal, liver, and adrenals of rats, were found to be similar, but Km values for l-DOPA were about 3-fold higher than those for l-5-HTP in the three tissues.