3'-Phosphoadenosine 5'-phosphosulphate:
galactosylceramide sulfotransferase which catalyzes the sulfation of
galactosylceramide (GalCer) was partially purified from a rat kidney light membrane fraction, and the properties were studied with special reference to substrate specificity. In order to study minimum molecular requirement of the acceptor substrate for the
sulfotransferase, analogues of
galactosylceramide where omega-amiocaproic or omega-aminododecanoic
acid is substituted for the acyl moiety of the native
glycolipid were chemically synthesized by improved procedures. The artificial
glycolipids were sulfated effectively by the kidney
enzyme, suggesting that the synthetic compounds will serve for affinity
ligands for the purification of the
enzyme. The extent of sulfation in the synthetic compounds was comparable with that of
galactosylceramide containing normal
acids and higher than that of galactosylsphingonine in which one of
hydrocarbon chains is deleted from the native
glycolipid. Through substrate specificity experiments, the
sulfotransferase has relatively broad specificity acting on beta-linked
galactosides at nonreducing ends of mono- and
disaccharides which bind, at least, one
hydrocarbon chain.
Enzyme kinetic analysis by competition assay using mixed acceptors demonstrated that the same, single
sulfotransferase catalyzes sulfation of
galactosylceramide,
lactosylceramide and
galactosylsphingosine. As to human
cancer, the
sulfotransferase activity was hardly detectable in
Wilms' tumor tissues that contrasts with
renal cell carcinoma tissues where the markedly elevated level was previously demonstrated. When sera from patients with various
cancers were examined for the
enzyme level, many cases of
hepatocellular carcinoma showed significantly increased activity, whereas the
hepatoma tissues had hardly detectable level of the
enzyme. These observations suggest that a humoral factor derived from the
hepatoma induces the
sulfotransferase.