A new form of residual hexosaminidase activity in infantile Tay Sachs disease fibroblasts.

Fibroblast cell lines obtained from five patients with the early onset form of Tay Sachs disease (TSD) possess a species of beta-N-acetylhexosaminidase (Hex) which is more anionic than Hex B but which is stable to heating under conditions which completely inactivate Hex A. This species, which comprised between 3 and 20% of the total hexosaminidase activity in homozygous TSD fibroblasts, appeared to be unstable and upon isoelectric focussing produced a mixture of Hex B (pI = 7.2) and an isozyme with a pI of 6.2. This intermediate form of hexosaminidase was not seen in two normal fibroblast cell lines but was observed following anion exchange chromatography of extracts of fibroblast cell lines obtained from two obligate heterozygotes. A species of hexosaminidase with the same chromatographic properties, thermostability and isoelectric point as the intermediate form found in fibroblasts with the TSD genotypes can be recovered after anion exchange chromatography of a partially purified preparation of human liver Hex A that had been treated with merthiolate. We hypothesize that in TSD cells a form of the beta subunit which is usually incorporated into Hex A accumulates due to the absence of alpha subunits. This form of the beta subunit is more anionic than the beta subunit found in Hex B. In the absence of alpha subunits these anionic beta subunits form tetramers with a pI = 6.2. This form of the enzyme is unstable in the presence of cellular proteases and may be modified to Hexosaminidase B.