Mapping of proteolytic and cyanogen bromide peptides from subunits of intestinal maltase–glucoamylase: evidence for significant homology

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Abstract
Rat intestinal maltase–glucoamylase was purified in the presence of detergent and proteolytic inhibitors, and the 130 000 and 145 000 subunits were separated and isolated by preparative sodium dodecyl sulfate – polyacrylamide gel electrophoresis and electrophoretic elution. Amino acid analyses were very similar, with a small excess of apolar amino acid residues in the 145 000 subunit. Peptide mapping with Staphylococcus aureus V8 protease revealed eight similar peptide products for each, with apparent elongation of the five larger peptides in the 145 000 subunit by a relative mass (Mr) of 2000–5000. α-Chymotrypsin maps showed at least eight identical cleavage products plus one large, shared product which was larger by a Mr of 5000 in the 145 000 subunit. Cyanogen bromide cleavage of the 145 000 subunit produced a single peptide of Mr 75 000. A peptide of Mr 66 000, also indicative of a central cleavage, was generated from the 130 000 subunit, but a second cleavage into 43 000 and 23 000 segments was also evident. Several sets of antibodies formed against both antigens consistently gave reactions of identity without spurring on immunodiffusion. These results indicate extreme homology between the central segment of the 145 000 subunit and the 130 000 subunit. The cyanogen bromide cleavage results suggest, however, that the two central sequences are not absolutely identical and therefore that one subunit may not be a posttranslational derivative of the other.