Specificity of rat liver plasma membrane serine/threonine protein kinases and phosphatases over endogenous proteins.

The specificity of rat liver plasma membrane protein kinases and phosphatases was examined over endogenous substrates, using specific effectors of these enzymes. cAMP-dependent protein kinase was shown to phosphorylate the 77, 60 and 51 kDa phosphoproteins and type II casein kinase, a specific 24 kDa one. On the contrary, types 1 and 2A protein phosphatases seemed to have a broad specificity in plasma membranes.

Stabilization of phospho-intermediates of rat liver plasma membrane alkaline phosphatase by uncompetitive inhibition. Relation with phosphate uptake into hepatocytes.

Rat liver plasma membrane alkaline phosphatase (ALP) phospho-intermediates, which have molecular masses of 151 and 135 kDa bands, were labelled at physiological pH with either (gamma-32P) ATP or 32Pi. This labeling was stabilized by a potent enzyme inhibitor, bromolevamisole (BL), and not by bromodexamisole (BD). BL augmented the rate and extent of autophosphorylation and slowed down the rate of autodephosphorylation of ALP.

Alkaline phosphatase activity at physiological pH: kinetic properties and biological significance.

The activity of rat liver alkaline phosphatase (ALP) was studied at physiological pH, using para-nitrophenyl phosphate (pNPP) as substrate. At this pH, the purified enzyme had optimal catalytic efficiency and its activity was maximal for the very low substrate concentrations. During thermal inactivation of rat liver plasma membranes activities, the ratio of the measured residual activities (pH 10.

Endogenous phosphorylation and dephosphorylation of rat liver plasma membrane proteins, suggesting a 18 kDa phosphoprotein as a potential substrate for alkaline phosphatase.

Purified rat liver plasma membranes were incubated for 0-60 min with [gamma-32P]ATP and analysis of 32P-labeled proteins by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed the presence of two shifted kinetic phenomena. The use of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), a potent inhibitor of protein kinases, allowed the identification of one as the endogenous protein phosphorylation. The other was shown to be the labeling of two phospho-intermediate forms of alkaline phosphatase (orthophosphoric monoester phosphohydrolase (alkaline optimum, EC 3.

Enhancement of mitogen-induced lymphocyte proliferation by some inhibitors of alkaline phosphatase and diamine oxidase.

We selected various compounds [bromolevamisole, levamisole, cimetidine, L-homoarginine, 2,3,5,6-tetrahydroimidazo-(2,1-b)thiazole (IT), imidazole, theophylline] previously reported as inhibitors of alkaline phosphatase (ALP) and/or diamine oxidase (DAO) and studied their activity on concanavalin A (ConA)-induced mouse spleen cell lymphocyte proliferation. According to the Ki values, the decreasing order of potency for ALP inhibition was: bromolevamisole, levamisole, theophylline, cimetidine, IT, imidazole and L-homoarginine. The order of potency was different for DAO inhibition.