Tyrosine hydroxylase in snail ( Helix pomatia) nervous tissue

Abstract

Snail nervous tissue synthesizes [ 14]dopamine and [ 14]dihydroxyphenylalanine (DOPA) from [ 14C]tyrosine. The K m value for the overall conversion of [ 14C]dopamine was 6 × 10 −4M. The enzyme converting [ 14C]tyrosine to [ 14C]DOPA. tyrosine hydroxylase, has the following characteristics. Approximately 85–90 per cent of the enzyme is soluble, and the enzyme of the nervous tissue, isolated by ammonium sulfate fractionation, had the highest activity in the 25–40 per cent fraction. The enzyme has a pH optimum of 6.5 in Tris-HCl, sodium acetate and potassium phosphate buffers. The enzyme requires a tetrahydropteridine cofactor. K m values toward various tetrahydropteridines such as 2-amino-4-hydroxy-6,7-dimethyltetrahydropteridine (DMPH 4), 2-amino-4-hydroxy-6-methyltetrahydropteridine (6MPH 4), and 2-amino-4-hydroxy-6-(L-erythro-1, 2-dihydroxypropyl)tetrahydropteridine ( l-erythro-tetrahydrobiopterin) (BH 4) are 5 × 10 −4 M. 3.2 × 10 −4 M and 1.1 × 10 −4 M respectively. The K m values for tyrosine at 10 −3 M BH 4 or 6MPH 4 are 1.4 × 10 −4 M and 4.2 × 10 −5 respectively. The enzyme is markedly stimulated by Fe 2+ and catalase. The activity is drastically inhibited by dopamine, 6-hydroxydopamine, 5-hydroxytryptophan (5-HTP), noradrenaline and sodium dodecyl sulphate. Analogues of tyrosine also slightly inhibit the activity while Triton X-100, homovanillic acid, dihydroxyphenylacetic acid (DOPAC), reserpine, tyramine, pargyline and sucrose have little effect. The properties of the snail tyrosine hydroxylase are compared with those of the vertebrates.