Perturbed epidermal pterin metabolism in Hermansky-Pudlak Syndrome

Abstract

In Hermansky-Pudlak Syndrome (HPS) a mutation in a 79.3 kDa transmembrane protein has been shown. The function of this protein has escaped definition so far. This study unveils a defective (6R)-L-erythro- 5,6,7,8-tetrahydrobiopterin (6BH4) de novo synthesis/recycling for this cofactor in HPS, where activities of the key enzyme GTP-cyclohydrolase I are in the normal range, but total biopterin levels are significantly decreased in homozygotes (n = 5) compared with unaffected controls (n = 4) (p = 0.00001). Phenylalanine hydroxylase and 4a-hydroxy-6BH4-dehydratase activities are significantly lower. mRNA of all enzymes involved in 6BH4 biosynthesis/recycling and GTP-cyclohydrolase I feedback regulatory protein were expressed in keratinocytes from homozygotes, heterozygotes, and healthy controls. Thioredoxin/thioredoxin reductase can directly control the redox status of 6BH4. These activities are allosterically controlled by calcium. Therefore calcium would directly affect this redox status. In HPS these enzyme activities are low concomitant with a defective calcium uptake, suggesting an extracellular accumulation of this second messenger. In this context phenylalanine hydroxylase is subject to phosphorylation/activation by calcium/calmodulin activated kinases. Therefore it was anticipated that calcium could directly affect the cellular L-phenylalanine turnover to L- tyrosine. A significantly more rapid L-phenylalanine uptake and its turnover to L-tyrosine was identified in normal human melanocytes (n = 5) and keratinocytes (n = 2), and was more enhanced in melanocytes in the presence of 2 x 10-3 M calcium. The turnover to L-tyrosine was significantly slower. Based on all evidence to date, we speculate that the mutated protein in HPS could be primarily involved in maintaining calcium homeostasis in this patient group.