Role of C6 chirality of tetrahydropterin cofactor in catalysis and regulation of tyrosine and phenylalanine hydroxylases.

The chiral specificities of bovine striatal tyrosine hydroxylase (TH) (unphosphorylated and phosphorylated by cAMP-dependent protein kinase) and rat liver phenylalanine hydroxylase (PH) were examined at physiological pH using the pure C6 stereoisomers of 6-methyl- and 6-propyl-5,6,7,8-tetrahydropterin (6-methyl-PH4 and 6-propyl-PH4) and (6R)- and (6S)-tetrahydrobiopterin (BH4). Both PH and phosphorylated TH have substantially higher Vmax values with the unnatural (6R)-propyl-PH4 than the natural (6S)-propyl-PH4 (approximately 6- and 11-fold, respectively). However, the Km's are also higher such that Vmax/Km is almost unaffected by C6 chirality. Unphosphorylated TH has equal Km values for both isomers of 6-propyl-PH4, but has about a 6 times greater Vmax with the unnatural isomer, making it the fastest cofactor yet for this form of the enzyme. With the shorter 6-methyl group, chiral differences are still recognized by phosphorylated TH but hardly at all by PH. Inhibition of both PH and TH by amino acid substrate which occurs with (6R)-BH4 as cofactor is also observed with (6S)-propyl-PH4 but not with (6S)-BH4, (6R)-propyl-PH4, or (6R)- or (6R,S)-methyl-PH4. The Km for (6S)-BH4 with phosphorylated TH is nearly 3 times higher than with (6R)-BH4, but Vmax is unchanged. With unphosphorylated TH, (6S)-BH4 produces very low decelerating rates, which was shown not to be due to irreversible inactivation of the enzyme. The Km for (6R)-BH4 with either hydroxylase is 10 times higher than for the equivalently configured (6S)-propyl-PH4. Comparison of these two cofactors reveals that the 1' and 2' side-chain hydroxyl groups of the natural cofactor promote different regulatory functions in PH than in TH.

Changes in the cofactor binding domain of bovine striatal tyrosine hydroxylase at physiological pH upon cAMP-dependent phosphorylation mapped with tetrahydrobiopterin analogues.

The structure of the cofactor binding domain of tyrosine hydroxylase (TH) was examined at physiological pH by determining kinetic parameters of (R)-tetrahydrobiopterin [(R)-BH4] and a series of tetrahydropterin (PH4) derivatives (6-R1-6-R2-PH4: R1 = H and R2 = methyl, hydroxymethyl, ethyl, methoxymethyl, phenyl, and cyclohexyl; R1 = methyl and R2 = methyl, ethyl, propyl, phenyl, and benzyl). A minimally purified TH preparation that was not specifically phosphorylated (designated as "unphosphorylated") was compared with enzyme phosphorylated with cAMP-dependent protein kinase. The Km for tyrosine with most tetrahydropterin analogues ranged between 20 and 60 microM with little decrease upon phosphorylation. Two exceptions were an unusually low Km of 7 microM with 6-ethyl-PH4 and a high Km of 120 microM with 6-phenyl-6-methyl-PH4, both with phosphorylated TH. Tyrosine substrate inhibition was elicited only with (R)-BH4 and 6-hydroxymethyl-PH4. With unphosphorylated TH (with the exception of 6-benzyl-6-methyl-PH4, Km = 4 mM) an inverse correlation between cofactor Km and side-chain hydrophobicity was observed ranging from a high with (R)-BH4 (5 mM) to a low with 6-cyclohexyl-PH4 (0.3 mM). An 8-fold span of Vmax was seen overall. Phosphorylation caused a 0.6-4-fold increase in Vmax and a 35-2000-fold decrease in Km for cofactor, ranging from a high of 60 microM with 6-methyl-PH4 to a low of 0.6 microM with 6-cyclohexyl-PH4. A correlation of the size of the hydrocarbon component of the side chain with affinity is strongly evident with phosphorylated TH, but in contrast to unphosphorylated enzyme, the hydroxyl groups in hydroxymethyl-PH4 (20 microM) and (R)-BH4 (3 microM) decrease Km in comparison to that of 6-methyl-PH4. Although 6,6-disubstituted analogues were found with affinities near that of (R)-BH4 (e.g., 6-propyl-6-methyl-PH4, 4 microM), they were frequently more loosely associated with phosphorylated TH than their monosubstituted counterparts (6-phenyl-PH4, 0.8 microM; cf. 6-phenyl-6-methyl-PH4, 8 microM). A model of the cofactor side-chain binding domain is proposed in which a limited region of nonpolar protein residue(s) capable of van der Waals contact with the hydrocarbon backbone of the (R)-BH4 dihydroxypropyl group is opposite to a recognition site for hydroxyl(s). Although interaction with either the hydrophilic or hydrophobic regions of unphosphorylated tyrosine hydroxylase is possible, phosphorylation by cAMP-dependent protein kinase appears to optimize the simultaneous operation of both forces.

Chronic mucocutaneous candidiasis. Immunologic studies of three generations of a single family.

A family consisting of eight members in three generations (age 10 months to 53 years) affected with chronic mucocutaneous candidiasis was studied along with three unaffected relatives. Dermatophytosis, loss of teeth and recurrent viral infections were present in some members. Results of tests for endocrinologic, muscle or liver disease, thymoma, iron deficiency, antitissue antibodies and malabsorption were normal in all patients. Antibody function and levels, B cell counts, serum complement, leukocyte enzymes, chemotaxis, phagocytosis and adherence were normal in all members. Plasma inhibitors to lymphocyte transformation and leukocyte inhibitory factor were not found. No unique HLA haplotype or antigen segregated in this family. Evaluation of cell-mediated immunity revealed total cutaneous anergy in three of eight whereas four of the other five had negative lymphocyte transformation and skin tests to Candida but responded normally to other antigens. Leukocyte inhibitory factor was not produced to Candida antigen in all four patients tested. T cell counts were within normal limits in all. Extensive evaluation of all limbs of the immune system in this family revealed a defect in cell-mediated immunity to Candida that appeared to be inherited as a dominant characteristic.