Affinity purification of anti-pigeon liver fatty acid synthetase immunoglobulin and comparative immunoreactivity of the catalytic reactions

Inhibition of pigeon liver fatty acid synthetase by specific modification of lysine residues with 2,4,6-trinitrobenzenesulphonic acid

Pigeon liver fatty acid synthetase was inactivated irreversibly by 2,4,6-trinitrobenzenesulphonic acid (TNBS). Biphasic inactivation of the enzyme was observed with the inhibitor. NADPH provided protection to the enzyme against inactivation by TNBS and the extent of protection increased with NADPH concentration indicating that the essential lysine residues are present at the NADPH binding site. The stoichiometric results with TNBS showed that 4 mol of lysine residues are modified per mole of fatty acid synthetase upon complete inactivation. The rapid reaction of two amino groups per enzyme molecule led to the loss of 60% of the enzyme activity. These approaches suggested that two lysine residues present at the active site are essential for the enzymatic activity of fatty acid synthetase.

Inactivation of enoyl-CoA reductase in pigeon liver fatty acid synthetase by pyridoxal 5'-phosphate: evidence for the presence of one lysine residue at the active site

Pigeon liver fatty acid synthetase (FAS) was rapidly inactivated by pyridoxal 5'-phosphate (PLP). Assays of the partial activities of the PLP-treated synthetase showed that only the enoyl-CoA reductase was decreased significantly. The inactivation of both the overall activity and enoyl-CoA reductase activity of FAS by PLP could be reversed by dialysis or dilution but not by reduction with sodium borohydride. Malonyl-CoA and acetyl-CoA did not protect the enzyme, whereas NADPH provided 68% protection against PLP-inactivation indicating that PLP modified lysine residues present at or near the co-enzyme binding site. PLP-treated enzyme after reduction with sodium borohydride exhibited fluorescence with a maximum at 397 nm (irradiation at 325 nm). Stoichiometric analysis showed that modification of four lysine residues per enzyme molecule resulted in complete inactivation of the overall and enoyl-CoA reductase activities of FAS. NADPH prevented the inactivation by protecting two of these lysine residues from modification, suggesting the presence of two essential lysine residues per enzyme molecule. These results are consistent with the hypothesis that each subunit of the enzyme contains an enoyl-CoA reductase domain in which a lysine residue, at or near the active site, interacts with NADPH.

Evidence for the essential histidine at the NADPH binding site of enoyl-CoA reductase domain of pigeon liver fatty acid synthetase

Pigeon liver fatty acid synthetase was inactivated by stoichiometric concentrations of diethylpyrocarbonate (DEP). The second order rate constant for the loss of synthetase activity was similar to the value for enoyl-CoA reductase indicating that ethoxyformylation destroys the ability of the enzyme to reduce the unsaturated acyl intermediate, without significant effect on beta-ketoacyl reductase activity. NADPH provided protection to the enzyme against inactivation by DEP indicating that essential histidine residues are present at the active site. DEP-modified enzyme showed a characteristic absorption maxima at 240 nm confirming the formation of ethoxyformic histidine. The reversal of inactivation by hydroxylamine and a pKa value of 7.0 obtained from the pH-rate profile for inactivation again confirmed the specificity of DEP for histidine. Stoichiometric results showed that two moles of histidine residue per mole of enzyme are essential for the activity of FAS.

Purification of nucleotide-requiring enzymes by immunoaffinity chromatography

Monospecific (affinity-purified) anti-(yeast glucose-6-phosphate dehydrogenase) IgG inhibits three different NADPH-requiring enzymes, chicken liver dihydrofolate reductase, pigeon liver fatty acid synthetase and chicken liver malic enzyme. The inhibition of all three enzymes was approx. 50% in a 2h incubation with 100 micrograms of IgG. Similarly, with several different NADH-requiring enzymes, an immunocrossreactivity was observed. Monospecific anti-(rabbit muscle glyceraldehyde-3-phosphate dehydrogenase) IgG inhibited yeast alcohol dehydrogenase and pig heart malate dehydrogenase by 39% and 55% respectively. The cross-reactivity observed was tested by affinity chromatography. Immunoaffinity columns made with each monospecific IgG were able to bind each of the enzymes it immunotitrated. Enzymes were eluted with a nondenaturing solvent with little loss of activity. The immunoaffinity column with monospecific anti-(glucose-6-phosphate dehydrogenase) IgG as the bound ligand was also used to purify partially (over 150-fold) both isocitrate dehydrogenase and dihydrofolate reductase from crude rat liver homogenate.

Fatty acid synthetase, malic enzyme and other NADP+ binding dehydrogenases have similar antigenic determinant(s) at the NADPH binding domain

Rabbit IgG prepared against malic enzyme inhibits pigeon liver fatty acid synthetase activity. By screening a number of polyclonal antibodies raised to several dehydrogenases, we have discovered that there is a direct relationship between this cross-reactivity and the existence of a common structural domain whose function is the binding of NADP+/NADPH. The presence of NADP+/NADPH in small concentrations protects against this immunoinhibition, thereby indicating a competitive binding of nucleotide and site-specific antibody at the antigenic domain. These data provide further support for the existence of an antigenic domain in the NADPH binding region of dehydrogenases which is responsible for the production of NADPH site-specific antibodies in polyclonal antisera.

Degradation of pigeon liver fatty acid synthetase in the absence of exogenous proteinases

The homogeneity of pigeon liver fatty acid synthetase has been rigorously tested by physicochemical techniques and crossed-rocket immunoelectrophoresis. The enzyme has also been incubated for 1 h at 100 degrees C in 2% sodium dodecyl sulfate and 0.1 M dithiothreitol. The number of protein components on gel electrophoresis and of dansylated amino acids increased as a function of incubation time. Furthermore, the minor proteins observed after gel electrophoresis cross-reacted with antibody raised to the synthetase. Proteolysis was not chemically mediated by the detergent, the reducing agent or the buffer conditions chosen. Several commercially prepared proteins were not degraded by this procedure, and two proteins were recalcitrant to hydrolysis when included in the same incubation mixture as the synthetase. The inclusion of certain microbial proteinase inhibitors decreased the amount of degradation. This demonstrated that hydrolysis of the synthetase is mediated by a specific vertebrate enzyme which retains activity under denaturing conditions at 100 degrees C. Further degradation is also observed after individual treatment of four limited digestion products from the pigeon liver fatty acid synthetase, suggesting the possibility of an inherent proteolytic activity within the complex.

Antibodies specific for NADPH-binding region of enzymes possessing dehydrogenase activities

The results reported in this paper show the presence of a population of antibodies in rabbit polyclonal antiserum that recognize an antigenic site at the NADPH-binding region of enzymes possessing dehydrogenase activities. Antisera from rabbits immunized with glucose-6-phosphate dehydrogenase or fatty acid synthetase were found to inactivate the enzyme dihydrofolate reductase. The inhibitory effect of this site-specific antibody is a time- and concentration-dependent reaction. This immunoinactivation is prevented by preincubation of the enzyme with NADPH.