Inactivation of rabbit liver and muscle aldolases by limited proteolysis by lysosomal cathepsin M

Nongenetic variation, genetic-environmental interactions and altered gene expression. III. Posttranslational modifications

The use of protein electrophoretic data for determining the relationships among species or populations is widespread and generally accepted. However, posttranslational modifications have been discovered in many of the commonly analyzed proteins and enzymes. Posttranslational modifications often alter the electrophoretic mobility of the modified enzyme or protein. Because posttranslational modifications may affect only a fraction of the total enzyme or protein, an additional staining band often appears on gels as a result, and this may confound interpretations. Deamidation, acteylation, proteolytic modification, and oxidation of sulfhydryl groups are modifications that often result in an electrophoretic mobility shift. Sialic acid-induced heterogeneity has been documented for many enzymes, but neuraminidase treatment can often remove sialic acids and produce gel patterns that are easier to interpret. In some cases, ontogenetic and tissue-specific expression may be due to posttranslational modifications rather than gene control and restricted expression, respectively. Methods of preventing, detecting and eliminating posttranslational modifications are discussed. Some posttranslational modifications may be useful for detecting cryptic genetic polymorphisms.

Purification and properties of rabbit liver cathepsin M and cathepsin B

Cathepsins M and B from rabbit liver lysosomes were separated by chromatography on Ultrogel AcA34 at low ionic strength and purified to homogeneity, and their catalytic and molecular properties were compared. Cathepsin M was relatively inactive with synthetic peptide substrates. Thus, it hydrolyzed benzoyl arginine naphthylamide at only one-fifth the rate observed with cathepsin B, and no activity was detected with Gly-Phe naphthylamide which is a relatively good substrate for cathepsin B. On the other hand, cathepsin M exhibited a preference for protein substrates. It was more active than cathepsin B in catalyzing the inactivation of the following enzymes: rabbit muscle or liver fructose-1,6-bisphosphate aldolases, rabbit liver fructose-1,6-bisphosphatase and pyruvate kinase, yeast glucose-6-phosphate dehydrogenase, and rabbit muscle glyceraldehyde-3-phosphate dehydrogenase. With glucagon as substrate, both enzymes showed similar peptidyl dipeptidase activities with some minor differences in peptide bond specificity. Cathepsins M and B are similar in size, with apparent molecular weights of 30,200 for cathepsin M and 28,800 for cathepsin B, and in amino acid composition and carbohydrate content. Each contains approximately 2-3 equivalents/mol glucosamine, 3 equivalents/mol mannose, and no fucose or galactosamine. They also show similar microheterogeneity in sodium dodecylsulfate-gel electrophoresis and isoelectric focusing; this microheterogeneity is probably related to differences in glycosylation. Extensive homology in primary structure for the two proteins was indicated by the similar patterns of peptides formed on digestion with trypsin.