INTERGENIC COMPLEMENTATION AFTER FUSION OF FIBROBLASTS FROM DIFFERENT PATIENTS WITH BETA-GALACTOSIDASE DEFICIENCY

Acid β-galactosidase from human liver consists, after gel filtration at pH 7.0, of a monomeric isoenzyme, β-galactosidase A and small amounts of a multimer, β-galactosidase B (Norden, A.G.W., Tennant, L.; and O'Brien, J.S. (1974) J. Biol. Chem. 249, 7969-7976). Our studies showed identical gel filtration patterns for β-galactosidase from human liver and cultured skin fibroblasts. Gel filtration in the buffer used for enzyme assays (pH 4.5) however, revealed primarily dimeric β-galactosidase, minor amounts of β-galactosidase A but no multimers. As the transitions between monomeric and dimeric β-galactosidase were reversible, dimeric β-galactosidase is apparently the enzymatically active isoenzyme. The isoenzymes from patients with four different clinical variants of β-galactosidase deficiency showed altered aggregation patterns, although their molecular weights corresponded to the normal molecular weights. It could be demonstrated that the restoration of β-galactosidase activity, which occurred after cell fusion of certain combinations of different β-galactosidase deficient fibroblasts, is not the result of intragenic complementation. Mixing experiments with cell-free extracts ruled out that the formation of dimeric β-galactosidase is required for complementation. To explain the complementation a model is suggested involving two different genes: a structural gene responsible for the synthesis of the polypeptide chain and a second gene which is involved in a modification process. © 1979.