Phosphoproteins

Iionization of tyrosyl groups of ovalbumin under native and denaturing conditions

Phenolic titration of ovalbumin was performed in the pH range 7-12 at 30 degrees C and at three ionic strengths viz. 0.033, 0.133 and 0.200. The conformational integrity of ovalbumin was studied by viscosity measurements at different pH values in the pH range 7-12.4. At ionic strength 0.133 two phenolic groups titrated reversibly with pKint = 10.31, and w = 0.032 up to pH 11.25 under native conditions. The value of w expectedly decreased with increase in ionic strength. Two additional phenolic groups became available for reversible titration between pH 11.25 and 11.95 after some conformational change. Above pH 12, the phenolic titration became irreversible and all of the nine tyrosine residues were titrated at pH 13.3 Exposure of ovalbumin to alkaline pH (12.4) caused considerable disruption of the native protein conformation. The reduced viscosity increased from 4.2 ml/g at pH 7.0 to 16.8 ml/g at pH 12.4 under identical conditions of the protein concentration. All of the nine tyrosyl groups of ovalbumin were titrated normally (pKint = 9.9) in a mixture of 5 M guanidine hydrochloride and 1.2 M urea. However, even in this mixture electrostatic interaction, as measured by w was not completely abolished.

Protein kinases from liver mitochondria of tumour-bearing rats

The mitochondria of liver of Yoshida ascites tumour-bearing rats contained two forms of protein kinase distinguishable on the basis of their kinetic properties, substrate specificity and responses to cyclic adenosine 3',5'-monophosphate (cAMP). One of these (kinase I) was activated 2-3 fold by cAMP while the other form (kinase II) was insensitive to the action of cAMP. Kinase I which was selective towards histone F1 as substrate was obtained as a homogeneous preparation and was observed to have a molecular weight of 170 000 by Sephadex G-150 gel filtration. Protein kinase II appeared to be a smaller protein with molecular weight of 54 000 and was specific towards acidic proteins namely casein and phosvitin. Protein kinases isolated from liver mitochondria of normal rats showed variations in respect to elution profile of DEAE-cellulose and electrophoretic mobility. The preparation corresponding to kinase I did not show stimulatory responses to cAMP.

Phospho-accepting proteins in bovine sera

Phospho-accepting proteins in bovine sera have been detected by the use of immobilized protein kinase from rat muscle and (gamma32P)-ATP in an in vitro system. A partial biochemical characterization points to the generation of typical phosphoproteins. Differences in the phosphorylation pattern between fetal serum and calf serum as demonstrated by electrophoresis in the presence of dodecylsulfate are described.

A kinetic analysis of the dephosphorylation, by bovine spleen phosphoprotein phosphatase (EC 3.1.3.16) of a phosphopeptide derived from beta-casein

A peptide containing the four closely grouped phosphoseryl residues present in beta-casein has been enzymatically dephosphorylated with bovine spleen phosphoprotein phosphatase (EC 3.1.3.16). The course of the dephosphorylation reaction has been followed by cellulose acetate electrophoresis and the amount of partially phosphorylated peptides present at each stage quantified by the same method. The phosphate groups are shown to be removed in a sequential manner and the rate constants for each stage of the dephosphorylation have been computed from the data obtained. The rate constants indicate that interaction in the intact peptide results in an enhancement of the activity of the phosphoseryl cluster.

Phosphorylated sites in substrates of intracellular protein kinases: a common feature in amino acid sequences

Examination of the primary amino acid sequences surrounding phosphorylated sites in many intracellular phosphoproteins indicated that the phosphorylated hydroxyamino acid (either serine or threonine) is, in general, surrounded by amino acids having a positively charged side chain and, more specifically, is frequently separated from a basic amino acid (either lysine or arginine) by only one amino acid. Possible reasons for this common feature are discussed.

31P nuclear-magnetic-resonance studies on the developing embryos of Xenopus laevis

The concentrations of nucleoside triphosphate, inorganic phosphate and the yolk proteins, phosvitin and lipovitellin, have been monitored in living embryos of Xenopus laevis by 31P nuclear magnetic resonance (NMR) spectroscopy. The nucleoside triphosphate levels remain relatively constant at about 3.5-4.5 nmol/embryo at least until the 'spontaneous movement' stage of development. By the swimming tadpole stage an inorganic phosphate resonance representing about 30 nmol/embryo becomes evident in the NMR spectrum. Computer manipulation also shows such a resonance, although smaller, to be present at a somewhat earlier developmental stage; these findings are confirmed biochemically. The major contribution to the NMR spectrum of oocytes, unfertilized eggs and early embryos is the yolk phosphoprotein resonance. On isolation of the yolk from the embryos it is possible to quantify the contribution to the NMR spectrum from the lipid-phosphate and protein-phosphate moieties of the yolk proteins. During development, as the yolk is used up, it is found that the protein-phosphate resonance disappears at a greater rate than the lipid-phosphate peak. The total phosphorus content of the embryo (approximately 200 nmol/embryo) is shown biochemically to remain constant during development; however, the total amount of phosphorus observed by NMR decreases by about 40% during development. From the resonance positions of their alpha, beta and gamma phosphate groups it is deduced that the nucleoside triphosphate molecules are liganded in vivo to a divalent cation which is not manganese, but could be either magnesium or calcium. From the position of the inorganic phosphate resonance it is deduced that the internal pH of embryos where this resonance is evident is 6.8 +/- 0.2.

Regulation of Membrane Flexibility in Human Erythrocytes

We have used spin-labels to detect prostaglandin E induced changes in erythrocyte membranes. The observed changes in spin-label resonance spectra can be mimicked in erythrocyte ghosts by loading them with cAMP or cGMP. These changes can also be observed by adding either of these cyclic nucleotides to intact cells. This entry of cyclic nucleotides into intact cells is blocked by an inhibitor of the anion channel. We suggest that the observed changes in paramagnetic resonance spectra are due to changes in lipid "fluidity" that are brought about by changes in the biochemical state of membrane-associated proteins (such as spectrin) and in the direct or indirect biophysical interactions of these proteins with membrane lipids.