The in situ kinetics of dopamine beta-hydroxylase in bovine adrenomedullary chromaffin cells. Intravesicular compartmentation reduces apparent affinity for the cofactor ascorbate.

The Km of dopamine beta-hydroxylase for its cofactor, ascorbic acid, was determined in situ in primary cultures of bovine adrenomedullary chromaffin cells and in isolated chromaffin vesicles. A range of intravesicular ascorbate concentrations in chromaffin cell cultures (1.1-31.2 mM) was achieved by varying the number and concentration of ascorbate additions to the culture media. The rate of octopamine synthesis from tyramine displayed a Michaelis-Menten relationship with respect to ascorbate concentration and an apparent Km of dopamine beta-hydroxylase for ascorbate of 15.0 +/- 2.0 mM was determined. In isolated chromaffin vesicles, with an initial intravesicular ascorbate concentration of approximately 10 mM, ascorbate consumption during beta-hydroxylation occurred as a first order process. This indicated that dopamine beta-hydroxylase was not saturated at this initial ascorbate concentration. When isolated chromaffin vesicles were prepared with different intravesicular ascorbate concentrations, the rate of octopamine synthesis displayed a Michaelis-Menten relationship with respect to ascorbate with an apparent Km of 17.0 +/- 5.0 mM. Ascorbate consumption also occurred as a first order process in ascorbate-loaded chromaffin-vesicle ghosts which had initial ascorbate concentrations of approximately 30 mM but which were depleted of other small molecules such as catecholamines. These results indicate that the in situ Km of dopamine beta-hydroxylase for ascorbate (approximately 15 mM) is 25-fold higher than it is for the purified or partially purified enzyme assayed under optimal conditions in vitro (0.6 mM). The factor(s) which decreases the enzyme affinity for ascorbate, relative to in vitro, resides in the chromaffin vesicle interior and is also retained in chromaffin-vesicle ghosts. The mechanism of this effect remains to be determined. The Km value determined in these experiments is close to the estimated intravesicular ascorbate concentration of bovine chromaffin granules in vivo (4), suggesting that the availability of ascorbate could become a factor in regulating the rate of dopamine beta-hydroxylation.