Human placental beta-galactosidase: structural and immunological observations.

beta-Galactosidase purified to apparent homogeneity from human placenta occurred in two separable fractions. A low molecular mass form (relative mass (Mr) 170 000) is composed of a single polypeptide chain (Mr 70 000). This was derived from a larger form by molecular sieve chromatography at both low (10 mM) and high (500 mM) NaCl concentration. The larger form of beta-galactosidase also contains small amounts of two polypeptides with apparent Mr values of 23 000 and 35 000 daltons. Both forms of the enzyme hydrolyze synthetic aryl galactosides and natural glycolipid substrates at comparable rates. Antibodies raised in rabbits against the low Mr beta-galactosidase also cross-reacts with the high Mr enzyme. The antibody preparation also cross-reacted with beta-hexosaminidase even though the latter was found at very low levels in the antigen, as judged by lack of detection of representative protein bands on sodium dodecyl sulfate - polyacrylamide gel electrophoresis and enzyme activity measurements. A portion of this cross-reactivity (35%) against beta-hexosaminidase could not be absorbed from the preparation without the simultaneous loss of beta-galactosidase activity, suggesting that the two enzymes show a degree of antigenic identity.