Spleen phosphoprotein phosphatase

Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography purification up to 40 000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 kDa and an isoelectric point of 7.9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it in a number of characteristics. 

The purple acid phosphatases (PAPs) are a class of phosphoprotein phosphatases which possess a p-oxo(hydroxo)-bridged dinuclear iron centre. An enzyme has been isolated from beef spleen which is purple in colour, while a violet phosphatase has been characterised from red kidney beans (KBPase). This latter enzyme consists of two subunits with M = 58200 and contains two equivalents of Zn(II) and Fe(III) per dimer which are essential for catalytic activity. KBPase hydrolyses nucleosidetriphos-phates as well as activated phosphomonoesters such as 4-nitrophenylphosphate or a-naphthyl phosphate (Beck et al., 1986). As with the beef spleen enzyme, KBPase is inhibited by tetrahedral oxoanions such PO and AsO . 

In a recent communication Sundararajan and Sarma report that a phosphoprotein phosphatase from rat spleen dephosphorylates a-, /3-, and unfractionated casein (90). Since these authors state that their enzyme differs in its action from that of a phosphomonoesterase, their results are in accord with the occurrence of a variety of phosphorus bonds in proteins. In this connection it should be noted that intestinal phosphatase used in our work at pH 9.0 also liberates all of the a-casein phosphorus (72). As discussed earlier, although this enzyme at pH 6.0 hydrolyzes —N—P—... 

The first evidence for a unique class of enzymes, transcending conventional phosphatase classification, came firom work on bovine spleen add phosphatase It was found that this phosphoprotein phosphatase was capable of removing the phosphorus from... 

A phosphoprotein phosphatase from beef spleen contains one atom of iron per molecule (Campbell and Zerner, 1973) and is activated by reducing agents. Similar iron-containing phosphatases have been isolated from Neurospora crassa (Jacobs et al, 1971) and kidney beans (Nochmunson et al, 1974). 

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