Abstract: Lipoamidase, an enzyme which removes lipoic acid from the e-amino group of a lysine residue in 2-oxoacid dehydrogenase complexes, has been measured in human serum and breast milk, and in different rat tissues. Serum from a boy with biotinid~e deficiency had low lipoarnidase activity measured as lipoyl-lysine hydrolase (LLH) and lipoyl-PABA hydrolase (LPH). Unexpectedly, LPH was not decreased in the same order as LLH and biotinidase, indicating that LPH in serum differs from the main lipoamidase.Lipoamidase measured as LLH and LPH was purified 20 000-fold to homogeneity from human serum, and after SDS/PAGE gave one band with an apparent molecular weight of 76 kDa. Lipoarnidase was eo-purified with biotinidase indicating that these enzyme activities belong to the same protein. After IEP in the presence of cysteine as thiol stabilizer two peaks were obtained, one of them probably being a cysteine adduct.The properties of the enzymes from a crude preparation of rat liver microsomes also indicated that lipoamidase measured as LLH belongs to the same enzyme protein as biotinidase, whereas LPH is another enzyme. These enzymes were solubilized from rat liver microsomes andseparated from each other by arrunonium sulphate fractionation. From these studies I conclude that lipoamidase activities in human serum and in rat liver are not derived from a genuine lipoamidase. Lipoamidase measured as LLH is the same enzyme as biotinidase and LPH is an enzyme different from biotinidase. Because we do not yet know the biological function of LPH, I prefer the names lipoyl-p-aminbenzoic acid hydrolase and lipoyl-X hydrolase.

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