Mammalian Phosphoprotein Phosphatase

Abstract

THE demonstration by Harris¹ and Barth et al.^2,3 of an enzyme in frog egg releasing phosphate from an endogenous phosphoprotein substrate was followed by the observation⁴ that mammalian tissue preparations split the phosphate bonds of exogenous phosphoproteins such as casein and phosvitin, at acid pH, at rates of about 60 µmoles phosphoprotein phosphorus/gm. wet wt. tissue/hr. Both these enzymic activities were attributed to ‘phosphoprotein phosphatases’. Studies on preparations from rat liver and spleen^4,5 described a Mg²⁺ and thiol-activated enzyme, labile to heat and salt fractionation, and active on casein and phosvitin but not on simple phosphate esters, thus distinguishing it from acid phosphomonoesterase. It was presumed to function t>y the splitting of the phosphoseryl bonds of phosphoproteins without accompanying proteolysis^4,5,21.