Chemical pathology of krabbe's disease. IV. Studies of galactosylceramide and lactosylceramide BETA-galactosidases in brain, white blood cells and aminotic fluid cells

The use of galactosylceramides with uniform fatty acids as substrates in the diagnosis and carrier detection of Krabbe disease

Cerebroside-beta-galactosidase (galactosylceramidase EC 3.2.1.4.6) activity was studied using galactosylceramides of uniform fatty acid composition. The highest activity and the best discrimination between patients with Krabbe disease and controls were found with N-nervonoylgalactosylsphingosine (C 24: 1-cerebroside). As a general rule cerebrosides with a monoenoic fatty acid gave higher activity and better discrimination than the corresponding cerebroside with a saturated fatty acid, the differences being largest for the cerebrosides with the longest fatty acids. In two methods the C 24: 1 cerebroside was used as substrate in the assay of the cerebroside-beta-galactosidase activity in leukocytes from 12 Krabbe patients, 14 parents and 22 controls. In a third method lactosylceramide prepared from mammalian brain gangliosides was used as substrate. With all three methods the residual activity in the leukocytes of the Krabbe patients did not exceed 5%, there was no tendency for overlap between the activities of the patients and those of the obligate carriers, and the values of half the carriers fell within the range for the controls.

Biochemical, genetic, psychometric, and neuropsychological studies in heterozygotes of a family with globoid cell leucodystrophy (Krabbe's disease)

Detection of a patient suffering from Krabbe's disease led to carrier screening in his family. Determination of galactosylceramide beta-galactosidase activity revealed the occurrence of two different alleles among the carriers of the same family. Heterozygotes and their noncarrier relatives were studied using psychometric and neuropsychological tests under blind conditions. It was found that compared to seven adult noncarrier relatives 19 adult carriers differ significantly in their general IQ and some subtests of the Wechsler Intelligence Scale for adults (WISA), including spatial cognition. Reaction times were significantly slower in the carriers with enzyme activity below 25% of the control values. Most of the carriers of this family have had myopia since early childhood.

Prenatal diagnosis of Krabbe disease

Krabbe disease was diagnosed prenatally in Göteborg (Sweden) and Lyon (France) by assaying the cerebroside-beta-galactosidase activity with galactosylceramides and lactosylceramides as substrates in cultivated amniotic fluid cells. Altogether, 48 pregnancies at risk were monitored between 1972 and 1980. Ten pregnancies at risk were terminated because of a predicted affection of the fetus. Biochemical examination of material available from 7 of the 10 abortuses confirmed the diagnoses. All the remaining 36 pregnancies ended in the birth of a healthy infant. The study showed that prenatal diagnosis of Krabbe disease is difficult because of the relatively high residual cerebroside-beta-galactosidase activity in some affected fetuses. Except for the large biological variation, the enzyme activity was sensitive to variation in cultivation conditions and differed strikingly between morphologically different cell types. These two factors were controlled by including control cell samples cultivated under identical conditions and by relating the cerebroside-beta-galactosidase activity to that of two marker enzymes. The biological variation was investigated further by measuring the cerebroside-beta-galactosidase activity in cultured skin fibroblasts from infants with Krabbe disease and from their parents. Results obtained in 18 unrelated patients with Krabbe disease, 26 obligate heterozygotes and 63 controls showed a wide range of variation in enzyme activity in the controls, a large overlap between the controls and obligate heterozygotes, and a high residual activity in some patients. Nevertheless, a high residual activity in a patient was combined with a relatively high enzyme activity in the two parents. In the light of the above findings and deliberations, it appears warranted to conclude that laboratories with experienced personnel can make a reliable prenatal diagnosis of Krabbe disease and that the examination should be offered to all known couples at risk.

Use of leukocytes in diagnosis of Krabbe disease and detection of carriers

The optimal conditions for the assay of the cerebroside-beta-galactosidase deficiency in Krabbe disease were studied. Labelled galactosylceramides and lactosylceramides isolated from natural sources were both suitable substrates provided that the lipophilic moieties were identical in labelled and unlabelled portions. Galactosylceramides and lactosylceramides with uniform fatty acid compositions were also tested. The highest specific cerebroside-beta-galactosidase activities were obtained with N-palmitoyl galactosyl- and lactosylceramides. The chromogenic substrate 2-hexadecanoyl-amino-4-nitrophenyl-beta-D-galactopyranoside (HNGal) was shown to be an unreliable substrate requiring large amounts of enzyme protein and giving false normal results. Krabbe disease was diagnosed in 45 patients. With galactosylceramides as substrates the residual cerebroside-beta-galactosidase activity was 7%, with lactosylceramides 8%, and there was no overlap in enzyme activity between the 45 patients and 42 parents. Approximately 50% of the obligate carriers had values within the range for the normal controls, independently of which substrate was used.

Specificity of galactosylceramidase activation by phosphatidylserine

Bovine brain phosphatidylserine effectively activates human brain galactosylceramidase (Hanada, E. and Suzuki, K. (1979) Biochim. Biophys. Acta 575, 410-420). Its effect on the other beta-galactosidase (Gm1-ganglioside beta-galactosidase) in human tissues, genetically distinct from galactosylceramidase, was examined. When partially purified human brain beta-galactosidase preparations, pure with respect to each other, were used as the enzyme source and when lactosylceramide, a common glycosphingolipid substrate for both beta-galactosidases, was used as the substrate, phosphatidylserine activated only hydrolysis of lactosylceramide by galactosylceramidase but not by GM1-ganglioside beta-galactosidase. With either galactosylceramide or lactosylceramide as substrate, and with phosphatidylserine as the activator, diagnosis of globoid cell leukodystrophy was possible using whole homogenates of cultured fibroblasts. Since 80-90% of lactosylceramide-cleaving activity in normal fibroblasts is due to GM1-ganglioside beta-galactosidase and since fibroblasts of globoid cell leukodystrophy patients are genetically deficient in galactosylceramidase but normal in GM1-ganglioside beta-galactosidase, these rsults are also consistent with specific activation of galactosylceramidase by phosphatidylserine.

The interrelations between high- and low-molecular weight forms of normal and mutant (Krabbe-disease) galactocerebrosidase

Galactocerebrosidase (beta-d-galactosyl-N-acylsphingosine galactohydrolase; EC 3.2.1.46) activity of brain and liver preparations from normal individuals and patients with Krabbe disease (globoid-cell leukodystrophy) have been separated by gel filtration into four different molecular-weight forms. The apparent mol.wts. were 760000+/-34000 and 121000+/-10000 for the high- and low-molecular-weight forms (peaks I and IV respectively) and 499000+/-22000 (mean+/-s.d.) and 256000+/-12000 for the intermediate forms (peaks II and III respectively). On examination by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, the high- and low-molecular-weight forms revealed a single protein band with a similar mobility corresponding to a mol.wt. of about 125000. Antigenic identity was demonstrated between the various molecular-weight forms of the normal and the mutant galactocerebrosidases by using antisera against either the high- or the low-molecular-weight enzymes. The high-molecular-weight form of galactocerebrosidase was found to possess higher specific activity toward natural substrates when compared with the low-molecular-weight form. It is suggested that the high-molecular-weight enzyme is the active form in vivo and an aggregation process that proceeds from a monomer (mol.wt. approx. 125000) to a dimer (mol.wt. approx. 250000) and from the dimer to either a tetramer (mol.wt. approx. 500000) or a hexamer (mol.wt. approx. 750000) takes place in normal as well as in Krabbe-disease tissues.

Activation of human brain galactosylceramidase by phosphatidylserine

Assays of sphingolipid hydrolases in vitro generally require bile salts or other detergents. A few 'activator proteins' have been reported that can partially replace the detergents in the assay mixture. We report here that phosphatidylserine from bovine brain is a relatively specific activator of human brain galactosylceramidase in the absence of sodium taurocholate (phosphatidylserine system). Activity similar to that obtained with the conventional assay system containing taurocholate and oleic acid (taurocholate system) could be obtained. Other lipids tested generally gave less than 10% of the taurocholate system activity, but sulfatide could activate human brain galactosylceramidase to 20--30% of the taurocholate system. The properties of the reaction in the phosphatidylserine system were examined with human brain whole homogenate, crude soluble post-concanavalin A preparations, and partially purified preparations as the enzyme source and compared with those obtained with the taurocholate system. The pH optimum shifted from 4.2 in the taurocholate system to 4.7 in the phosphatidylserine system. The phosphatidylserine system was superior in the linearity of the reaction with respect to the enzyme protein. Reasonably linear Lineweaver-Burk plots could be obtained. The Km values for the phosphatidylserine system were greater than those for the taurocholate system. The effect of phosphatidylserine was not additive to that of taurocholate. Additional phosphatidylserine to the taurocholate system was either without effect at lower concentrations or inhibitory at higher concentrations. The assays of galactosylceramidase with phosphatidylserine and without taurocholate do not necessarily provide pragmatic advantages but offer a potentially useful system with which to study the mechanism of in vivo degradation of the membrane-bound glycosphingolipid.

Basic findings and current developments in sphingolipidoses

Sphingolipidoses are caused by recessively inherited deficiencies of lysosomal hydrolases. The clinical backgrounds of and current biochemical and genetic approaches to the different forms and variants of gangliosidoses, trihexosylceramidosis (Fabry's disease), galactosylceramidosis (Krabbe's disease), sulfatidoses (metachromatic leukodystrophies), glucosylceramidosis (Gaucher's disease), sphingomyelinoses (Niemann-Pick disease) and ceramidosis (Farber's disease) are presented.

Use of a chromogenic substrate for the diagnosis of Krabbe's disease, with special reference to its application in prenatal diagnosis

A chromogenic substrate, 2-hexadecanoylamino-4-nitrophenyl-beta-D-galactopyranoside, has recently been described for the diagnosis of Krabbe's disease. Hydrolysis of this substrate by extracts of cultured cells and tissues was compared with the activities of lactocerebrosidase I and non-specific beta-galactosidase. Under appropriate conditions, hydrolysis of the chromogenic analogue was markedly reduced in extracts of cultured amniotic fluid cells and skin fibroblasts derived from cases of Krabbe's disease. Activity was also markedly deficient in extracts of Krabbe's brain, although only a partial reduction was measured in liver extracts. Generally activities were higher in tissues of fetal origion. Unfortunately, the new analogue proved less specific and less sensitive than the natural substrates used to diagnose Krabbe's disease. Consequently, the analogue does not provide a satisfactory alternative substrate for the prenatal diagnosis of Krabbe's disease.

Antenatal diagnosis of Krabbe's leucodystrophy: enzymatic and morphological confirmation in an affected fetus

Galactosylceramide beta-galactosidase activity was assayed in cultured amniotic fluid cells from two pregnancies at risk for Krabbe's leucodystrophy. The elective termination of one pregnancy was carried out after demonstration of a severe deficiency of galactosylceramide beta-galactosidase activity. The diagnosis of Krabbe's leucodystrophy in the affected fetus was confirmed enzymatically by a deficiency of galactosylceramide beta-galactosidase inbrain, liver, kidney, and cultured skin fibroblasts, and histologically by the ultrastructural demonstration of the typical intracellular inclusions in cells of the spinal cord.

Diagnosis of the sphingolipidoses with labelled natural substrates

The genetic heterogeneity of sphingolipidoses is underlined and the desirability of using natural labelled substrates for the diagnoses of each new index case strongly emphasized. Recent studies of our Scandinavian Krabbe families (more than 50) have repeatedly shown that there is no method developed which can be used for the detection of carriers of the mutant gene in leukocytes or lymphocytes. Also described are enzymic studies in two forms of Gaucher disease which further demonstrate the importance of natural substrates for the diagnoses of the disease in leukocytes and cultivated amniotic fluid cells.

Substrate specificities of the two genetically distinct human brain beta-galactosidases

The two human brain beta-galactosidases were solubilized and fractionated by Sephadex G-200 gel filtration, free from each other. Substrate specificities of the two enzymes were examined for galactosylceramide, lactosyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]galactosyl-glucosylceramide (GMI-ganglioside), galactosyl-N-acetylgalactosaminyl-galactosyl-glucosylceramide (asialo GM1-ganglioside), and 4-methylumbelliferyl beta-galactoside. Under appropriately optimized conditions, either of the two beta-galactosidases could hydrolyze all of the substrates, although with widely varying rates. Relative specific activities of galactosylceramide beta-galactosidase toward galactosylceramide, lactosyl-[N-steroyl]ceramide, lactosyl-[N-lignoceroyl]ceramide. GM1-ganglioside, asialo GM1-ganglioside, and 4-methylumbelliferyl beta-galactoside were 100, 510, 250, 39, 41 and 120, respectively. Relative specific activities of GM1-ganglioside beta-galactosidase toward the same series of the substrates were 0.3, 78, 19, 100, 150 and 240; However, the optimal assay conditions for any given natural substrate were sufficiently different for each beta-galactosidase so that diagnostic assays for the two genetic diseases due to beta-galactosidase deficiencies could be carried out in whole tissues. Since the relative distribution of the two enzymes vary greatly in different tissues, contributions by the two enzymes to degradation of the natural glycosphingolipids in vivo may well vary in different organs. These findings may have an important bearing on the biochemical pathogenesis of these genetic disorders.

Effect of bile salts on lactosylceramide beta-galactosidase activities in human brain, liver and cultured skin fibroblasts

The effect of bile salts on the hydrolysis of lactosylcermide by human beta-galactosidases in vitro was studied using cultured skin fibroblasts, liver and brain tissue. The evidence for two distinct enzymes that can catalyze the hydrolysis of lactosylceramide was observed when the bile salt was changed from pure sodium taurocholate to either crude taurocholate, or pure glycodeoxycholate, taurodeoxycholate or taurochenodeoxycholate. Tissues from patients with Krabbe's disease were found to be deficient in lactosylceramide beta-galactosidase activity (lactosylceramidase I) when pure taurocholate was used in the assay. When crude taurocholate was used in the assay, the Krabbe patients appeared to have normal activity for this enzyme. In place of crude taurocholate the pure salts of glycodeoxycholate, taurodeoxycholate and taurochenodeoxycholate worked even better to stimulate the second lactosylceramide beta-galactosidase activity and GM1 gangliosidosis patients exhibiting little if any activity. Therefore, lactosylcermidase I is stimulated by crude taurocholate or pure glycodeoxycholate, taurodeoxycholate and taurochenodeoxycholate. The use of pure bile salts to assay lactosylceramidase I and II will result in better reproducibility for these enzyme activities between laboratories.