Gaucher disease (GD; glucosylceramidosis) is caused by a deficient activity of the enzyme glucocerebrosidase (GC). Clinical manifestations are highly variable and cannot be predicted accurately on the basis of the properties of mutant GC. Analysis of secondary abnormalities, such as elevated plasma levels of some hydrolases, may help to increase insight into the complicated pathophysiology of the disease and could also provide useful disease markers. The recent availability of enzyme supplementation therapy for GD increases the need for markers as early predictors of the efficacy of treatment. We report the finding of a very marked increase in chitotrisidase activity in plasma of 30 of 32 symptomatic type 1 GD patients studied: the median activity being > 600 times the median value in plasma of healthy volunteers. In three GC-deficient individuals without clinical symptoms, only slight increases were noted. Chitotriosidase activity was absent in plasma of three control subjects and two patients. During enzyme supplementation therapy, chitotriosidase activity declined dramatically. We conclude that plasma chitotriosidase levels can serve as a new diagnostic hallmark of GD and should prove to be useful in assessing whether clinical manifestations of GD are present and for monitoring the efficacy of therapeutic intervention.

Fabry disease is an X-linked lysosomal storage disease caused by deficiency of α-galactosidase A that affects males and shows disease expression in heterozygotes. The characteristic progressive renal insufficiency, cardiac involvement, and neuropathology usually are ascribed to globotriaosylceramide accumulation in the endothelium. However, no direct correlation exists between lipid storage and clinical manifestations, and treatment of patients with recombinant enzymes does not reverse several key signs despite clearance of lipid from the endothelium. We therefore investigated the possibility that globotriaosylceramide metabolites are a missing link in the pathogenesis. We report that deacylated globotriaosylceramide, globotriaosylsphingosine, and a minor additional metabolite are dramatically increased in plasma of classically affected male Fabry patients and plasma and tissues of Fabry mice. Plasma globotriaosylceramide levels are reduced by therapy. We show that globotriaosylsphingosine is an inhibitor of α-galactosidase A activity. Furthermore, exposure of smooth muscle cells, but not fibroblasts, to globotriaosylsphingosine at concentrations observed in plasma of patients promotes proliferation. The increased intima-media thickness in Fabry patients therefore may be related to the presence of this metabolite. Our findings suggest that measurement of circulating globotriaosylsphingosine will be useful to monitor Fabry disease and may contribute to a better understanding of the disorder.

Chitinases are ubiquitous chitin-fragmenting hydrolases. Recently we discovered the first human chitinase, named chitotriosidase, that is specifically expressed by phagocytes. We here report the identification, purification, and subsequent cloning of a second mammalian chitinase. This enzyme is characterized by an acidic isoelectric point and therefore named acidic mammalian chitinase (AMCase). In rodents and man the enzyme is relatively abundant in the gastrointestinal tract and is found to a lesser extent in the lung. Like chitotriosidase, AMCase is synthesized as a 50-kDa protein containing a 39-kDa N-terminal catalytic domain, a hinge region, and a C-terminal chitin-binding domain. In contrast to chitotriosidase, the enzyme is extremely acid stable and shows a distinct second pH optimum around pH 2. AMCase is capable of cleaving artificial chitin-like substrates as well as crab shell chitin and chitin as present in the fungal cell wall. Our study has revealed the existence of a chitinolytic enzyme in the gastrointestinal tract and lung that may play a role in digestion and/or defense.

The human chitinase, named chitotriosidase, is a member of family 18 of glycosylhydrolases. Following the cloning of the chitotriosidase cDNA (Boot, R. G., Renkema, G. H., Strijland, A., van Zonneveld, A. J., and Aerts, J. M. F. G. (1995) J. Biol. Chem. 270, 26252–26256), the gene and mRNA have been investigated. The chitotriosidase gene is assigned to chromosome 1q31-q32. The gene consists of 12 exons and spans about 20 kilobases. The nature of the common deficiency in chitotriosidase activity is reported. A 24-base pair duplication in exon 10 results in activation of a cryptic 3′ splice site, generating a mRNA with an in-frame deletion of 87 nucleotides. All chitotriosidase-deficient individuals tested were homozygous for the duplication. The observed carrier frequency of about 35% indicates that the duplication is the predominant cause of chitotriosidase deficiency. The presence of the duplication in individuals from various ethnic groups suggests that this mutation is relatively old.

In various mammals, enzymatically active and inactive members of family 18 glycosyl hydrolases, containing chitinases, have been identified. In man, chitotriosidase is the functional chitinolytic enzyme, whilst the homologous human cartilage 39‐kDa glycoprotein (HC gp‐39) does not exhibit chitinase activity and its function is unknown. This study establishes that HC gp‐39 is a chitin‐specific lectin. It is experimentally demonstrated that a single amino acid substitution in the catalytic centre of the 39‐kDa isoform of chitotriosidase, which generates a similar sequence to that in HC gp‐39, results in a loss of hydrolytic activity and creates the capacity to bind to chitin. The possible implication of the finding for chitinolytic and chitin‐binding proteins that are produced in high quantities by activated macrophages are discussed.