Gaucher disease, resulting from the decreased activity of the lysosomal enzyme 
glucocerebrosidase, is the most prevalent sphingolipid storage disease. Due to 
considerable heterogeneity of phenotypic expression, it has been subdivided into 
the nonneurological type 1 disease, and types 2 and 3, the neurological types. 
We describe homozygosity for the D409H mutation within the glucocerebrosidase 
gene associated with a unique form of type 3 Gaucher disease. Twelve patients, 
originating from three Arab sibships, were found to be homozygous for the D409H 
mutation. They all presented with oculomotor apraxia and a progressive cardiac 
valve defect with minimal organomegaly. When expressed in human cells in tissue 
culture, using the T7/EMC/vaccinia virus hybrid expression system, we were able 
to demonstrate that the mRNA carrying the D409H mutation was less stable than 
the normal counterpart. Pulse-chase experiments demonstrated that the mutated 
protein exhibited lower stability than the normal counterpart. Its activity 
toward the artificial substrate 4-methyl umbelliferyl glucopyranoside was 
similar to that of the mutated enzymes carrying the N370S or the L444P 
mutations. However, in loading experiments using lissamine-rhodamine conjugated 
glucosyl ceramide as a substrate, the recombinant mutated protein carrying the 
D409H mutation exhibited 28.63 +/- 6.05% of the activity exhibited by the normal 
enzyme. L444P and N370S mutations exhibited 51.90 +/- 7.16 and 115.75 +/- 12.64% 
of normal enzyme activity, respectively. Loading of cells homozygous for the 
D409H mutation demonstrated 10. 05% of the activity shown by normal cells. L444P 
and N370S homozygous cells demonstrated 25.3 and 98.5% of foreskin fibroblast 
glucocerebrosidase activity, respectively. We demonstrate that homozygosity for 
the D409H mutation is a unique case of a peculiar phenotype associated with a 
specific intracellular glucocerebrosidase activity.