We have cloned, sequenced, and expressed cDNAs encoding wild type human glutaryl-CoA dehydrogenase subunit, and have expressed a mutant enzyme found in a patient with glutaric acidemia type I. The mutant protein is expressed at the same level as the wild type in Escherichia coli, but has less than 1% of the activity of wild-type dehydrogenase. We also present evidence that the glutaryl-CoA dehydrogenase transcript is alternatively spliced in human fibroblasts and liver; the alternatively spliced mRNA, when expressed in E.coli, encodes a stable but inactive protein. Purified expressed human glutaryl-CoA dehydrogenase has kinetic constants similar to those of the previously purified porcine dehydrogenase. The primary translation product from in vitro transcribed glutaryl-CoA dehydrogenase mRNA is translocated into mitochondria and processed in the same manner as most other nuclear-encoded mitochondrial proteins. Human glutaryl-CoA dehydrogenase shows 53% sequence similarity to porcine medium chain acyl-CoA dehydrogenase, and these similarities were utilized to predict structure-function relationships in glutaryl-CoA dehydrogenase.