The molybdenum cofactor, a complex of Mo with an organic molecule termed molybdopterin, is a common component of the three human molybdoenzymes sulfite oxidase, xanthine dehydrogenase and aldehyde oxidase. The severe neurological defects and neonatal death observed in sulfite oxidase deficiency attests to the essentiality of the metal and molybdopterin for normal human development. Recent compilations of genome sequences of diverse organisms have revealed the existence of a universally common pathway for the biosynthesis of molybdopterin. The broad goals of this proposal are to identify the molecular lesions leading to sulfite oxidase deficiency or molybdenum cofactor deficiency. The defective genes will be identified in fibroblasts of individual patients, and sequenced to identify the genetic alteration. In the case of sulfite oxidase, the naturally occurring mutations leading to a single amino acid replacement will be reproduced in the cloned human gene. Heterologous expression of the human protein in Escerichia coli will enable analysis of the effect of the mutation on the physical and chemical properties of the enzyme. There are at least six proteins implicated in the synthesis and processing of the molybdenum cofactor, and the genes for all of them have now been identified and sequenced. MOCS1A and MOCS1B are involved in the formation of precursor Z from a guanosine derivative; while MCOS2A and MOCS2B together known as molybdopterin synthase, convert precursor Z to molybdopterin. MOCS3, known as synthase sulfurylase, charges the synthase with the sulfur atom ultimately found in molybdopterin. Gephyrin, a protein previously localized in post-synaptic vesicles and implicated in the clustering of several types of synaptic receptors, also plays a role in some aspects of the utilization of the molybdenum cofactors. Using appropriate primers, fibroblasts from cofactor-deficient patients will be sequenced to identify the particular protein affected in each case. These studies will facilitate understanding the complex chemical reactions leading to the formation of usable molybdenum cofactor.
