Cell membrane Phosphohydrolases

The phosphohydrolases are a group of enzymes which have been studied most extensively from the viewpoint of their association with membranes. For example, ATPase is regarded as a cell membrane component concerned with electrolyte transport glucose-6-phosphatase is a cell en-... 

Accordingly from the evidence presented and from the analogies drawn, one can justifiably formulate a picture of a variety of phosphohydrolase enzymes and of nonphosphohydrolase enzymes, including synthetic enzymes, incorporated along with phospholipid into cytoplasmic and cell membranes. 

Exopolyphosphatase activities were found and characterized in the purified preparations of yeast cell nuclei (Lichko et al., 1996, 2002b) and mitochondrial membranes (Lichko et al, 1998). The presence of specific exopolyphosphatases in these sub-cellular fractions was confirmed by their insensitivity to inhibitors of other phosphohydrolases occurring in the cell compartments and differences in pH optima. 

Particularly relevant in this discussion is acid phosphatase which is widely considered to be exclusively a lysosomal enzyme. Evidence from this laboratory will be presented which indicates that acid phosphatase is a component of membranes in epithelial cells of mouse kidney and that lysosomal and microsomal acid phosphatases are two different isoenzymes. With this demonstration, all of the phosphohydrolases discussed in this chapter do exist in a membrane-associated form among others. 

Since 1957, evidence has been accumulating that 5 -nucleotidase is a phosphohydrolase which resides in plasma membranes of hepatic parenchymal cells in mammalian species (Wachstein and Meisel, 1957). Upon fractionation of liver homogenates, 5 -nucleotidase is found in the microsomal fraction (Song et al., 1967). 

Not only are phosphohydrolases membrane components, but a variety of other enzymes such as glycosidases have been shown to share the same location. For example, the microvillar membranes of the absorbing epithelial cells of the intestine contain sucrase, maltase, and leucine amino-peptidase the basal infolding membranes of mouse renal epithelial cells exhibit both acid phosphatase and yS-glucuronidase. 

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