The activation of coagulation factor X. Identity of cleavage sites in the alternative activation pathways and characterization of the COOH-terminal peptide

Bovine Factor X can be activated by two alternative pathways. The first, favored at high concentrations of the complex of tissue factor and Factor VII, is initiated by the action of Factor VII on Factor X to cleave an activation peptide from the NH2 terminus of the heavy chain, to produce alpha-Xa. This is then converted autocatalytically to another form of Factor Xa, beta-Xa, by the loss of a 17-residue glycopeptide from the COOH terminus of the heavy chain, in a lipid-dependent reaction. The alternative pathway, favored at lower activator concentrations, is initiated by the action of Factor Xa on Factor X, in the presence of lipid, to release the same COOH-terminal peptide as is produced in the conversion of alpha-Xa to beta-Xa. The intermediate produced by the loss of this peptide from Factor X,I1, can be activated directly to beta-Xa by the tissue factor-Factor VII complex, with the loss of the same NH2-terminal peptide as is produced in the conversion of Factor X to alpha-Xa. The autocatalytic activation of Factor X by Factor Xa described previously occurs to a marked extent only at very low activator concentrations, and has been shown to proceed largely by the loss of the normal NH2-terminal peptide from the heavy chain of I1-Initial experiments show that neither peptide affects the rate of coagulation by either the extrinsic or intrinsic pathways. The amino acid sequences have been determined on both sides of the peptide cleavages, and it has been shown that the cleavage sites are the same, regardless of the pathway of activation. The amino acid sequence and carbohydrate composition of the COOH-terminal peptide have been determined. The carbohydrate moiety is attached via an O-glycosidic linkage at a threonine residue, and contains galactosamine but no glucosamine.

Chemical modifications of functional residues of fd gene 5 DNA-binding protein

The binding of gene 5 protein from bacteriophage fd to poly[d(A-T)], fdDNA, and poly(A) is accompanied by a dramatic reversal in the signs of the large ellipticity bands of the nucleic acid chromophores from 250 to 290 nm. The change in the circular dichroism of the DNA induced by the protein, which reaches a maximum at a protein to nucleotide molar ratio of 1:4, has been used as an assay of the alterations in binding of gene 5 protein to DNA accompanying changes in the ionic environment and subsequent to chemical modification of the protein. Divalent cations completely dissociate the gene 5 protein-fd DNA complex at 0.1 M, while 0.5 M monovalent cations are required. All cations are more effective in dissociating the complex with poly[d(A-T)] commensurate with the accompanying stabilization of the double helix to which gene 5 protein does not bind. Acetylation of all six lysyl residues and three of the five tyrosyl residues of the protein with N-acetylimidazole prevents complex formation. Removal of the three tyrosyl O-acetyl groups with hydroxylamine does not restore the binding of gene 5 protein to DNA. Tetranitromethane nitrates the same three tyrosyl residues (Tyr-26, Tyr-41, and Tyr-56 as determined by peptide mapping) and reduces the binding affinity of the protein for fd DNA by similar to 100-fold. The 19F NMR spectrum of gene 5 protein labeled with m-fluorotyrosine shows three surface and two buried fluorotyrosyl residues. All tyrosyl residues are protected from nitration in the complex with fd DNA, but acetylimidazole acetylates surface lysyl residues in the complex and dissociates it. The intrinsic circular dichroism of the acetylated and nitrated gene 5 proteins is not significantly altered. In contrast maleic anhydride reacts with the seven amino groups of the protein and changes the secondary structure to one similar to that present in 6 M guanidine-HCl. The single SH group of the native protein does not react with Ellman's reagent, but it reacts rapidly with one Hg2+ ion which unfolds the protein; fd DNA prevents reaction with Hg2+. Electrostatic forces may be as important as hydrogen bonding in maintaining the native structure of this protein. The lysyl groups of the protein, exposed in both the free protein and the DNA complex, appear to be of prime importance in DNA binding, probably through electrostatic interactions with the DNA binding, probably through electrostatic interactions with the DNA phosphate groups. Three tyrosyl residues also contribute to binding affinity through hydrogen bonding or intercalation. A model of gene 5 protein structure in relation to interactions with a tetranucleotide is presented.

Photo-incuced cross-linkage of gene-5 protein and bacteriophage fd DNA+

The gene 5 protein, coded for by the bacteriophage fd, forms a complex with single stranded fd-DNA such that one gene 5 protein monomer interacts with four bases. Exposure of this complex to ultraviolet light results in the formation of covalent bonds between 25-30% of the gene 5 protein monomers which are bound to the DNA. In contrast, when the intact fd virion, which is a complex of coat protein and DNA, was exposed to ultraviolet irradiation, no detectable protein DNA cross-links were found.

Effects of various cerebral metabolic activators on glucose metabolism of brain

The effects of various metabolic activators on the glucose metabolism of the perfused cat brain have been investigated. (1) When the perfusion is done with the blood containing cytidine monophosphate, the glucose metabolism of the brain is enhanced, as compared to the perfusion with the blood without cytidine monophosphate. There is no marked change in the cerebral metabolic rate and in the contents of intermediate metabolite of glucose in the brain. (2) Citicoline enhances the incorporation of blood glucose into the brain and its metabolism in the brain. It increases slightly the cerebral blood flow rate and decreases the accumulation of lactate in the brain. (3) Either Pyrithioxin or Meclophenoxate has no effect on the glucose metabolism of the brain nor on the cerebral metabolic rate.