ACTIVITIES OF D-XYLOASCORBIC AND L-XYLOASCORBIC ACID AND D-ARABOASCORBIC AND L-ARABOASCORBIC ACID AS A COFACTOR FOR DOPAMINE-BETA-HYDROXYLASE REACTION

L-Xyloascorbic acid (L-xylo-AsA) and its three stereoisomers, D-xyloas corbic acid (D-xylo-AsA), L-araboascorbic acid (L-arabo-AsA) and D-ara boascorbic acid (D-arabo-AsA), have been considered to show some diffe rences in vitamin C activity. In this paper the effect of L-xylo-AsA, D-xylo-AsA, L-arabo-AsA and D-arabo-AsA on the activity of dopamine be ta-hydroxylase was studied to clarify whether or not the structural sp ecificities of these stereoisomers have different effects on enzyme ac tivity. The maximum velocity (Vmax) of the hydroxylation and Km for as corbic acid were calculated using double-reciprocal plotting. Vmax for L-xylo-AsA was estimated to be 201 nmol/min/mg protein and those of D -xylo-AsA, L-arabo-AsA and D-arabo-AsA were 157 nmol/min/mg protein, 1 12 nmol/min/mg protein and 194 nmol/min/mg protein, respectively. Km f or L-xylo-AsA was 1.5 mM and those for D-xylo-AsA, L-arabo-AsA and D-a rabo-AsA were 2.3 mM, 2.7 mM and 1.4 mM, respectively. The effect of D -arabo-AsA on the activity of dopamine beta-hydroxylase was almost the same as that of L-xylo-AsA, while D-xylo-AsA and L-arabo-AsA showed s maller effects. Our results suggest that the configuration at carbon 4 might be more important than that of the hydroxyl group at carbon 5 f or the development of the activity as a cofactor for dopamine beta-hyd roxylase reaction.