Occurrence of DOPA/5-HTP Decarboxylase in Human Seminal Plasma: Kinetic Parameters and Demonstration of a Naturally Occurring Inhibitor of the Enzyme

¹ Département de Biochimie, Faculté de Médecine, Université Laval and Laboratoire d’Endocrinologie et Metabolisme, Le Centre Hospitalier de l’Université Laval, Québec, Canada

The authors found that DOPA/5-HTP decarboxylase was present in the human seminal plasma and that its activity was lower in fertile men than in infertile men. The Km and Vmax of the enzyme were, respectively, 0.88 x l0^-6 M and 19.8 pmol of serotonin/mg of protein/min. The enzyme was pyridoxal-5'-phosphate (PLP)-independent, and the content of PLP in the seminal plasma was lower in fertile than in infertile subjects. A soluble, heat stable, diffusible inhibitor of DOPA/5-HTP decarboxylase was detected in seminal plasma. This naturally occurring inhibitor was capable of reducing the activity of decarboxylase in the human seminal plasma and in cat liver. Zinc inhibited DOPA/5-HTP decarboxylase in both human seminal plasma and cat liver, suggesting the involvement of zinc, in close relationship with PLP, in the decarboxylation mechanism. These findings suggest a relationship between the synthesis of biogenic amines and testicular function in infertile men.

     Key words: DOPA/5-HTP decarboxylase, human seminal plasma, biogenic amines

Submitted on November 8, 1979
Revised on February 18, 1980
Accepted on February 25, 1980