Strain differences in kinetic and thermal stability of two mouse brain tryptophan hydroxylase activities

Levels of forebrain serotonin (5-HT), tryptophan, 5-hydroxyindoleacetic acid (5-HIAA) and hydroxylase cofactor (BH₄) were comparable in two experimental mouse strains (A/J and C57B1/6J) despite 2–3-fold differences in vitro in the relative activities of forebrain and midbrain tryptophan-5-monooxygenase (TPOH; EC 1.14.16.4). The enzyme activities did not differ with respect toK_m for cofactor at saturating levels, but manifested different degrees of cooperativity with respect to cofactor when examined with BH₄ concentrations within a physiological range. They differed also in the frequency and amplitude of kinetic variation around comparable mean velocity slopes across cofactor and time; in resistance to pre-incubation inactivation and responsiveness to its facilitation by calcium; in molecular weight heterogeneity as reflected in the distribution of molecular weight forms by gel diffusion chromatography; and in the number of peaks in power spectral analysis of kinetic variation patterns. Although the potential roles of small-molecule ligand and/or regulator proteins have not been ruled out, we hypothesize that differences in conformational stability underlie the differences in regulatory properties and make one enzyme activity more vulnerable to occlusive influences in vivo.