Kinetic properties of bovine pineal tryptophan-5-monooxygenase activated by an endogenous activating substance.

We previously reported that an endogenous activating substance different from bovine serum albumin, phospholipids and heparin, exists in the extract from bovine pineal glands and that this substance interacts with tryptophan-5-monooxygenase under reducing conditions with sulfhydryl reagents, to stimulate monooxygenase activity. The present paper reports that the activating substance is of peptide nature; that it is sensitive to trypsin-digestion; and that it does not change the apparent Km's for substrates, L-tryptophan and oxygen, and coenzyme, reduced biopterin or DMPH4; but that it increases the Vmax 1.5- to 2.3-fold. These results suggest that an activating protein, present in some particles of the cell structure, activates tryptophan-5-monooxygenase under the regulation of a sulfhydryl compound. The apparent Km's for reduced biopterin and DMPH4 were 77.2 microM and 294 microM, respectively. The apparent Km's for L-tryptophan and oxygen with reduced biopterin were 15.0 microM and 4.7%, respectively; with DMPH4, they were 11.0 microM and 8.5%, respectively. Significant inhibition of both L-tryptophan and oxygen was observed with reduced biopterin, but not with DMPH4 (at the tested concentrations of up to 0.5 mM and 20%, respectively).