Biochemistry and immunochemistry of membrane-bound enzymes

Membrane-bound enzymes have certain specific differences compared with soluble enzymes. Membrane-binding often enables greater catalytic activity of associated enzymatic reactions, their regulation by low molecular weight substances (substrates and allosteric effectors, hormones) and compartmentation, etc. On the other hand, the binding of enzymes to membranes causes considerable difficulties as regards their isolation and the determination of their homogeneity and substrate specificity. Membrane enzymes provide a unique opportunity for studying the biogenesis of membranes and their physiological properties, however. These problems are discussed in relation to two types of membranes--the inner mitochondrial membrane and the membrane of the brush border of the small intestine. An example of the utilization of immunochemical methods is given in the results of a study of biosynthesis of the cytochrome oxidase complex in yeast cells. In the case of the brush border of the mammalian small intestine, the fact that certain enzymes, which are also of clinical significance from the aspect of congenital genetic defects, can be isolated only as complexes, constitutes a very real problem. This applies particularly to the sucrase-isomaltase complex and the lactase-beta-glucosidase complex. Solving questions of substrate specificity is of significance for the choice of a suitable analytical or histochemical method. The common regulation of these complexes gives an insight into the problems of membrane biogenesis, however. Immunochemical methods can be employed as sensitive criteria to support biochemical and morphological studies. Collaboration between the biochemist and histochemist proved especially valuable when determining the substrate specificity of enzymes (glycosidases) in relation to histochemical substrates, when applying histochemical methods for detecting enzymatic activity in immunoprecipitates and acrylamide gels and in immunohistochemical studies of the localization and developmental differentiation of the enzymes of the brush border of the small intestine.