The major beta-hexosaminidase isozymes in humans are Hex A (deficient in Tay-Sachs disease, TSD), an alphabeta heterodimer and Hex B (deficient in Sandhoff disease) a betabeta homodimer. Hex S, the alphaalpha homodimer is physiologically unstable. Mature alpha and beta subunits share 60% sequence identity. The beta subunit active site hydrolyzes neutral substrates. The alpha subunit active site hydrolyzes neutral (4MUG) and charged substrates (4MUGS, GM2 ganglioside). Only Hex A hydrolyzes the natural substrate, GM2 ganglioside, in the presence of the GM2 activator protein (AP). / We investigated regions of the alpha and beta subunits involved in AP binding, subunit dimerization, and substrate specificity. We constructed four chimeric cDNAs: alpha1--259beta292--544 , alpha1--118beta152--544, beta 1--418alpha387--529, and beta1--151 alpha119--259beta292--544 (subscripts refer to amino acid residues). Chimeric cDNAs were expressed in a TSD neuroglial cell line, which produces no endogenous alpha subunits. The chimeric isozymes were chromatofocused and assayed for hydrolysis of (a) 4MUG, (b) 4MUGS and (c) GM2 ganglioside. / Transfection of the cDNA constructs lead to expression of homodimeric and heterodimeric chimeric proteins, albeit at lower yields than transfection of wild alpha-cDNA. All of the chimeric proteins hydrolyzed 4MUG but none were active towards 4MUGS or GM2 ganglioside. These results suggest that (a) all constructs contained sufficient information to form both heterodimeric and homodimeric chimeric proteins, (b) the chimeras lacked the alpha-subunit sequence necessary for the hydrolysis of charged substrates.

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As homeostatic organisms, rotifers have to use the mechanism to cope with nutrition unbalance in their food. The regulation of digestive enzyme activities as a possible physiological mechanism involved in maintaining of rotifer homeostasis was studied. This study further explored the effect of food quantity and quality on rotifer population growth rate and reproduction.