Dimerization and activation of porcine pancreatic phospholipase A2 via substrate level acylation of lysine 56

The porcine pancreatic phospholipase A2-catalyzed hydrolysis of the water-soluble chromogenic substrate 4-nitro-3-octanoyloxybenzoate shows an initial latency phase similar to the one observed in the hydrolysis of aggregated phospholipids by the same enzyme. We report here that during the latency phase the enzyme undergoes a slow, autocatalytic, substrate-level acylation whereby in a few of the catalytic events the scissile octanoyl group of the substrate, normally transferred to water, is transferred to the epsilon-amino group of lysine 56. The N epsilon 56-octanoylphospholipase shows a strong tendency to dimerize in solution and thus may be separated from the monomeric native enzyme by gel filtration. Octanoylation of Lys-56 activates the enzyme some 180-fold toward 4-nitro-3-octanoyloxybenzoate and more than 100-fold toward monolayers of 1,2-didecanoyl-sn-glycero-3-phosphocholine. Acylation also attends the enzymatic hydrolysis of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine with the incorporation of 1 eq of palmitate. Kinetic analysis of the early phase of reaction with 4-nitro-3-octanoyloxybenzoate shows that in this initial step the rate of activation is first order with respect to enzyme and substrate. A much more rapid, autocatalytic activation occurs in the later phases of the reaction where the activation of the enzyme is catalyzed by the activated enzyme itself. These findings with porcine pancreatic phospholipase A2, together with those relative to a snake venom enzyme monomer (Cho, W., Tomasselli, A. G., Heinrikson, R. L., and Kézdy, F. J. (1988) J. Biol. Chem. 263, 11237-11241), strongly support the proposal that interfacial activation of monomeric phospholipases is due to substrate-level autoacylation resulting in fully potentiated dimeric enzymes.     

A 113-amino acid fragment of CD4 produced in Escherichia coli blocks human immunodeficiency virus-induced cell fusion

A gene encoding a 113-amino acid, NH2-terminal fragment of CD4, rsT4.113, was constructed and expressed in Escherichia coli under the control of the tryptophan operon promoter. Following induction, rsT4.113 is produced at 5-10% of total E. coli protein, and it is found in inclusion bodies. The protein is purified in two steps under denaturing and reducing conditions. Solubilized rsT4.113 is first purified on a column of Q-Sepharose to remove low molecular weight contaminants and then purified to greater than 95% homogeneity by gel filtration. Renaturation of rsT4.113 is achieved at approximately 20% yield by dilution and dialysis. High performance liquid chromatography analysis of renatured rsT4.113 reveals a less than 15% contaminant of reduced protein. Purified and renatured rsT4.113 contains epitopes for both OKT4a and Leu3a, anti-CD4 monoclonal antibodies which block CD4-gp 120 association, but lacks measurable affinity toward a nonblocking anti-CD4 monoclonal antibody, OKT4. By comparison to a longer form (375 amino acids) of recombinant soluble T4 produced in mammalian cells that contains the entire extracellular domain, rsT4.113 has a comparable affinity for binding to OKT4a and Leu3a in a radioimmunoassay. Analysis of antiviral activity of rsT4.113 demonstrates that the E. coli-derived protein inhibits human immunodeficiency virus-induced syncytium formation with an IC50 of 5-10 micrograms/ml. These data demonstrate that the human immunodeficiency virus-binding domain of CD4 is localized within the NH2-terminal 113 amino acids of CD4 and is contained within a structure homologous to the kappa variable-like domain of immunoglobulins.     

Molecular phenotypes in cultured maple syrup urine disease cells. Complete E1 alpha cDNA sequence and mRNA and subunit contents of the human branched chain alpha-keto acid dehydrogenase complex

The activity of the branched-chain alpha-keto acid dehydrogenase complex is deficient in patients with the inherited maple syrup urine disease (MSUD). To elucidate the molecular basis of this metabolic disorder, we have isolated three overlapping cDNA clones encoding the E1 alpha subunit of the human enzyme complex. The composite human E1 alpha cDNA consists of 1783 base pairs encoding the entire human E1 alpha subunit of 400 amino acids with calculated Mr = 45,552. The human E1 alpha and the previously isolated human E2 cDNAs were used as probes in Northern blot analysis with cultured fibroblasts and lymphoblasts from seven unrelated MSUD patients. The results along with those of Western blotting have revealed five distinct molecular phenotypes according to mRNA and protein-subunit contents. These consist of type I, where the levels of E1 alpha mRNA and E1 alpha and E1 beta subunits are normal in cells, but E1 activity is deficient; Type II, where the E1 alpha mRNA is present in normal quantity, whereas the contents of E1 alpha and E1 beta subunits are reduced; Type III, where the level of E1 alpha mRNA is markedly reduced with a concomitant loss of E1 alpha and E1 beta subunits; Type IV, where the contents of both E2 mRNA and E2 subunits are markedly reduced; and Type V, where the E2 mRNA is normally expressed, but the E2 subunit is markedly reduced or completely absent. Type V includes thiamin-responsive (WG-34) and certain classical MSUD cells. These molecular phenotypes have demonstrated the complexity of MSUD and identified the affected gene in different patients for further characterization.     

Regulation of DNA supercoiling in Escherichia coli: genetic basis of a compensatory mutation in DNA gyrase

Bacterial DNA supercoiling is controlled by balancing the supercoiling activity of DNA gyrase and the relaxing activity of DNA topoisomerase I. We have characterized the gyrB gene from a top A deletion mutant of Escherichia coli (DM800) that has a compensatory mutation in gyrB, lowering the activity of gyrase 10-fold, and thereby redressing the intracellular level of supercoiling. The mutant gene differs from the wild type in carrying three rather than two direct tandem repeats of a 6 bp sequence encoding Ala-Arg. We suggest this novel mutation affects domain spacing and was generated by an unequal crossing over event, possibly involving gyrase.     

Glutamine synthetase gene of Bacillus subtilis

The glutamine synthetase gene (glnA) of Bacillus subtilis was purified from a library of B. subtilis DNA cloned in phage lambda. By mapping the locations of previously identified mutations in the glnA locus it was possible to correlate the genetic and physical maps. Mutations known to affect expression of the glnA gene and other genes were mapped within the coding region for glutamine synthetase, as determined by measuring the sizes of truncated, immunologically cross-reacting polypeptides coded for by various sub-cloned regions of the glnA gene. When the entire B. subtilis glnA gene was present on a plasmid it was capable of directing synthesis in Escherichia coli of B. subtilis glutamine synthetase as judged by enzymatic activity, antigenicity, and ability to allow growth of a glutamine auxotroph. By use of the cloned B. subtilis glnA gene as a hybridization probe, it was shown that the known variability of glutamine synthetase specific activity during growth in various nitrogen sources is fully accounted for by changes in glnA mRNA levels.     

Phenylalanine hydroxylase in liver cells. Correlation of glucagon-stimulated enzyme phosphorylation with expressed activity.

Phenylalanine is transported rapidly into, but is not concentrated by, liver cells. Glucagon increased flux through phenylalanine hydroxylase; a half-maximal response was obtained at 0.7 nM. Under control conditions, 0.2-0.3 mol of phosphate were incorporated per mol of subunit of the hydroxylase at steady state. Glucagon increased this incorporation of phosphate into the hydroxylase to a maximal value of approx. 0.6 mol of phosphate per subunit; a half-maximal response was obtained at 0.3 nM. Glucagon, added simultaneously with [32P]Pi to liver cells, inhibited incorporation of 32P into the enzyme. The effects of glucagon were reproduced with dibutyryl cyclic AMP. Changes in phosphorylation correlated closely with changes in flux through phenylalanine hydroxylase in cell incubations.     

The influence of starvation and tryptophan administration on the metabolism of phenylalanine, tyrosine and tryptophan in isolated rat liver cells.

Liver cells from fed Sprague-Dawley rats metabolized phenylalanine, tyrosine and tryptophan at rates consistent with the known kinetic properties of the first enzymes of each pathway. Starvation of rats for 48 h did not increase the maximal activities of phenylalanine hydroxylase, tryptophan 2,3-dioxygenase and tyrosine aminotransferase in liver cell extracts, when results were expressed in terms of cellular DNA. Catabolic flux through the first two enzymes was unchanged; that through the aminotransferase was elevated relatively to enzyme activity. This is interpreted in terms of changes in the concentrations of 2-oxoglutarate and glutamate. Cells from tryptophan-treated animals exhibited significant increases in the catabolism of tyrosine and tryptophan, but not of phenylalanine. The activities of tyrosine aminotransferase and tryptophan 2,3-dioxygenase were also increased, although the changes in flux and enzyme activity did not correspond exactly. These results are discussed with reference to the control of aromatic amino acid catabolism in liver; the role of substrate concentration is emphasized.