The inhibitor-sensitive sites of galactosyl ceramide galactosidase

Abstract

A preparation of galactosyl ceramide galactosidase from rat brain has been tested with potential inhibitors which resemble the enzyme's substrate. The amide made from 2-hydroxydodecanoic acid and -erythro-3-phenyl-2-amino-1,3-propanediol proved to be a fine inhibitor, acting noncompetitively. Removal of any of the three hydroxyl groups reduced the effectiveness, as did inversion of the C-3 carbon atom or addition of substituents on the benzene ring. N-Acetyl psychosine was an effective inhibitor of the mixed type while the longer homolog, N-decanoyl psychosine, was a competitive inhibitor as well as substrate for the enzyme. Lactosyl ceramide, the naturally occurring lipid, was a competitive inhibitor of modest efficacy. Galactonolactone was an excellent competitive inhibitor and N-(n-hexyl) psychosine was an active inhibitor of the mixed type. It would appear from the above comparisons that the enzyme's active site binds only galactose-containing or galactose-resembling substances, while the secondary effector site binds a variety of substances which possess the central nitrogenous core region of cerebroside.