In view of recent conflicting reports from two laboratories, activities of
lactosylceramide beta-galactosidase were reinvestigated in detail in brains and livers of normal individuals and of patients with
globoid cell leukodystrophy or GM1-gangliosidosis. Both sets of the apparently totally contradictory results were readily reproduced simply by using the different assay systems of the respective laboratories. With our own assay system, hepatic
lactosylceramide beta-galactosidase appeared deficient only in Gm1-gangliosidosis, while it appeared deficient only in
globoid cell leukodystrophy when the assay system of Wenger et al. (Wenger, D.A., Sattler, M., Clark, D., and McKelvey, H. (1974) Clin. Chim. Acta 56, 199-206) was used. Analyses of individual constitutents in the two assay systems revealed their complex effects on measured activities of the
enzyme. The findings were strongly indicative of the existence of two genetically distinct
lactosylceramide-cleaving
enzymes. One
enzyme (
lactosylceramidase I) may be identical with
galactosylceramide betal-
galactosidase, and the other (
lactosylceramidase II) is closely related to nonspecific 4-methylumbelliferyl
beta-galactosidase. Normal human brain contains mostly
lactosylceramidase I, while normal liver contains predominantly
lactosylceramidase II.
Lactosylceramidase I is genetically lacking
globoid cell leukodystrophy, and
lactosylceramidase II in GM1-gangliosidosis.
Lactosylceramidase I is activated by either pure or crude
taurocholate and by
oleic acid and is only slightly activated by
chloride ions.
Lactosylceramidase II is activated by crude
taurocholate but not by pure
taurocholate. As activators,
oleic acid is less effective and
chloride more effective than for
lactosylceramidase I.
Citrate-
phosphate buffer is more favorable to
lactosylceramidase I than
citrate buffer, while
lactosylceramidase II responds in reverse. The standard assay system used by Wenger et al. determines almost exclusively
lactosylceramidase I, while our own standard system is optimal for
lactosylceramidase II and is less favorable for
lactosylceramidase I. With a highly purified human hepatic
beta-galactosidase preparation, exxentially free of
galactosylceramide beta-galactosidase activity,
lactosylceramide-cleaving activity determined by the Wenger system was less than 2 per cent of that determined by our system. If
lactosylceramide beta-balactosidase assays are to be used for diagnosis of
globoid cell leukodystrophy, it is absolutely essential to use an appropriate assay system in order to avoid errors of serious consequences.