CHARACTERISATION OF DOPA DECARBOXYLASE

Abstract

This chapter discusses characteristics of DOPA decarboxylase. DOPA decarboxylase, which catalyses the conversion of L-dopa to dopamine, was the first of the catecholamine biosynthetic enzymes to be discovered. In a study, the dopa decarboxylase of hog kidney was purified to homogeneity. The final preparations were approximately 300-fold purified compared to the initial 25,000g supernatant and were essentially homogeneneous by polyacrylamide disk gel electrophoresis and analytical ultracentrifugation. The purified enzyme was found to require at least one free sulfhydryl group for activity. Although, the purified enzyme was associated with about 0.9 mole of pyridoxal 5′-phosphate (PLP) per mole of enzyme protein, maximal activity was attained only when exogenous cofactor was present. It was found that the purified enzyme decarboxylated dopa, 5-hydroxytryptophan (5-HTP), tryptophan, p-tyrosine and (very feebly) histidine. The molecular weight of the enzyme was 112,000 daltons by sedimentation equilibrium. Gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol gave rise to three bands of protein corresponding to molecular weights of about 57,000, 40,000 and 21,000 daltons.