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Biomedical subjects

L A Pinna

Publications and source records attributed to L A Pinna.

At least 199 records · Page 11Linked to original sources
PubMed1980

Protein phosphatase from rat liver nuclei.

Protein phosphatase, active on non-histone phosphoprotein substrate, was partially purified from rat liver cell nuclei by means of salt extraction, ammoniumsulfate precipitation, DEAE cellulose chromatography, gel filtration and preparative isoelectrofocusing. Rat liver nuclei contain a heterogenous population of different protein phosphatases. All the enzyme fractions eluted from DEAE cellulose are of low molecular weight between 12,000--31,000. The pH 5.5 peak fraction of preparative isoelectrofocusing was characterized in detail. It has a pH optimum of 6.8 using nuclear phosphoprotein substrate. It is inhibited by Na+ at 80 mM, and to a lesser extent by K+, activated by Mg2+ (5 mM) and Mn2+ (1 mM). However, the latter is inhibitory at 6 mM. The nuclear protein phosphatase is also active on labelled F1 and F2b histones and casein, however, its V is lower on histones and it contains component(s) active specifically on nuclear phosphoprotein substrate but not on casein.

Animals↗
PubMed1979

Phosphorylation of threonine and serine residues of native and partially dephosphorylated caseins by a rat liver cyclic AMP-insensitive protein kinase.

The preferential phosphorylation of threonine residues of native casein fractions by a rat liver cyclic AMP-independent protein kinase (EC 2.7.1.37) is abolished by preliminary limited dephosphorylation of the substrates, which promotes a fall in the phosphothreonine/phosphoserine ratios from values higher than 1 to much less than 0.1. This finding and the identification of the threonine residues phosphorylated support the view that the liver protein kinase affects threonine residues only when suitable serine residues, which fulfil the structural requirements for attack by the enzyme but which are not yet phosphorylated, are not available.

Animals↗
PubMed1979

The use of soybean trypsin inhibitors as phosphorylatable substrates for a rat liver protein kinase.

Phosphorylation by a cAMP-independent rat liver protein kinase of protein substrates containing the structural feature required by mammary gland casein kinase (-Ser-X-Glu/Asp) has been demonstrated. In particular, the Bowman-Birk Soybean trypsin inhibitor, which is characterized, like other legume protease inhibitors, by clusters of acidic residues near the C-terminal side of seryl residue(s), proved to be a good model substrate for the protein kinase. Its phosphorylation, involving the Ser 65 residue, is apparently hindered by the binding of trypsin, while it is stimulated by unfolding induced by reduction and subsequent carboxy-methylation.

Animals↗
PubMed1978

Different susceptibility of whole casein components to enzymatic phosphorylation by two forms of rat liver 'casein kinase'.

The phosphorylation of the single casein subfractions occurring when whole casein is incubated with [gamma-32P]ATP in the presence of two different rat liver 'casein kinases', both cyclic AMP-insensitive, has been studied. "Casein kinase TS", active on both threonine and serine residues of whole casein, was found to be active towards a minor protein fraction, running slightly ahead of beta-casein during gel electrophoresis, and accounting for most, if not all, of the [32P]Thr residues labeled in whole casein ("[32P]Thr-rich fraction"). The [32P]Ser residues labeled by this enzyme were recovered in an heterogeneous "[32P]Ser-rich fraction" including alphas1-casein together with minor alphas fractions, following alphas1-casein during gel electrophoresis. "Casein kinase S", on the other hand, active only towards serine residues of whole casein, is active almost exclusively towards the minor alphas casein fractions, with the exclusion of both the "[32P]Thr-rich fraction" and alphas1-casein itself. Therefore, of the major casein components, beta- and K-caseins apparently play a quite unimportant role in the overall phosphorylation of whole casein by both the protein kinases tested, while alphas1-casein itself, unlabeled by casein kinase S, accounts for no more than 20--30% of 32P incorporated in the presence of casein kinase TS.

Animals↗
PubMed1976

Comparative study of mitochondrial and cytosol protein kinase activities.

Both cytosol and mitochondria of rat liver display protein kinase activity, cyclic AMP-independent, which is resolved by Sepharose 6B filtration and P-cellulose chromatography into multiple forms phosphorylating, besides endogenous mitochondrial membrane-bound proteins, also exogenous phosphoproteins such as casein and phosvitin. However, the forms by far predominant in the cytosol phosphorylate both phosphorylserine and phosphorylthreonine residues of casein, while most of the activity associated to mitochondrial structures is due to the forms phosphorylating only phosphorylserine residues.

Animals↗
PubMed1976

Involvement of polyphosphorylserine blocks in the Fe(III) binding by phosvitin.

It has been suggested that the binding of iron(III) by phosvitin involves the phosphoric radicals of phosphorylserine residues, many of which are arranged in rows of several consecutive phosphoamino acids. In this paper we present evident that, unlike free phosphorylserine which does not interact with Fe(III), polyphosphorylserine blocks--(Ser-P)n, with n greater than or equal to 4 -- bind Fe(III) like phosvitin, though less actively, to give complexes stable at very acidic pHs. The binding of iron does not cause any polymerization of the phosphopeptides, and the (Ser-P)n-Fe(III) complexes display an Fe/P ratio significantly lower than 0.5, found in Fe-saturated phosvitin. These findings indicate that polyphosphorylserine blocks play an important role in the binding of iron by phosvitin, and that in the intact protein their binding capacity is optimized by the conformation of the polypeptide chain.

Binding Sites↗