Abstract
In the process of studying the inhibition of dopamine-β-hydroxylase by cysteine, we observed an interesting relationship between cysteine and catalase. This suggests that two different patterns of the inhibition of dopamine-β-hydroxylase exist; the one is reversible inhibition, the other is irreversible inhibition. In the present experiments, we have particularly investigated the irreversible inhibition of dopamine-β-hydroxylase by cysteine, and the following conclusions were drawn: (a) The rate of the inhibitory effect of cysteine pretreatment on dopamine-β-hydroxylase is independent of a function of the temperature, while at temperature above 20°C the extent of the reversibility of cysteine inhibition by N-ethylmaleimide is temperature dependent. (b) The rate of inhibitory effect is dependent on the time of cysteine pretreatment at 37°C, and the ability of N-ethylmaleimide to reverse the cysteine-induced inhibition is gradually diminished by increasing the time of cysteine pretreatment. (c) The inhibitory effect of cysteine pretreatment (37°C, 10min) is not reversed by the addition of Cu++. (d) In the presence of higher concentrations of catalase, the cysteine-induced inhibition is recovered after the addition of equimolar concentration of N-ethylmaleimide. (e) Both inhibitions of cysteine on dopamine-β-hydroxylase with and without pretreatment were the noncompetitive type to the substrate, tyramine. (f) Similar inhibitory effects can be observed in case of ascorbate, but not of glutathione.
