RECYCLING DEFECTS OF COVALENTLY-BOUND VITAMINS

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Biochemical and genetic studies of vitamin-responsive enzymopathies have resulted in the elucidation of a group of successfully treatable inherited metabolic disorders and have given new insight into the nutritional role of the various vitamins. We have recently demonstrated that one such disease, late-onset biotin-responsive multiple carboxylase deficiency, is caused by a deficiency of biotinidase activity. This enzyme catalyzes the removal of covalently bound biotin from the various coenzyme-dependent carboxylases, thus regenerating the vitamin for reutilization. To fully understand the genetic and nutritional significance of the covalently bound vitamins it is now clear that their metabolism as well as their mode of action must be reconsidered. Normal human biotinidase, purified to homogeneity, will be characterized by biochemical and immunological methodologies. Various biochemical aspects of normal biotinidase, its other biological functions and subcellular localization will be studied to better elucidate the enzyme's role in biotin metabolism and potential sites for genetic defects in other biotin-responsive disorders. We will characterize the biochemical and molecular basis of biotinidase deficiency using various immunological and molecular biological methods. These results may explain the clinical heterogeneity observed in this disorder and allow more accurate determination of heterozygotes for biotinidase deficiency. Finally, we will conduct biochemical studies of the enzyme, lipoamidase, which releases another covalently bound vitamin, lipoic acid. Because pyruvate dehydrogenase is a lipoic acid-dependent enzyme and deficiency of this enzyme is known to result in lactic acidosis, a primary defect in lipoamidase may be responsible for some unexplained disorders of lactic acid metabolism. The proposed research will provide a more complete understanding of the metabolic role of enzymes that recycle vitamins and their contribution to normal nutrition and a variety of known and potential vitamin deficiency states.