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2,4-Dihydroxyquinoline (DHQ) is an abundant extracellular metabolite of the opportunistic pathogen Pseudomonas aeruginosa that is secreted into growth medium in stationary phase to concentrations comparable with those of the Pseudomonas quinolone signal. Using a combination of biochemical and genetic approaches, we show that PqsD, a condensing enzyme in the pqs operon that is essential for Pseudomonas quinolone signal synthesis, accounts for DHQ formation in vivo. First, the anthraniloyl moiety is transferred to the active-site Cys of PqsD to form an anthraniloyl-PqsD intermediate, which then condenses with either malonyl-CoA or malonyl-acyl carrier protein to produce 3-(2-aminophenyl)-3-oxopropanoyl-CoA. This short-lived intermediate undergoes an intramolecular rearrangement to form DHQ. DHQ was produced by Escherichia coli coexpressing PqsA and PqsD, illustrating that these two proteins are the only factors necessary for DHQ synthesis. Thus, PqsD is responsible for the production of DHQ in P. aeruginosa.

Analogues of the natural antibiotic thiolactomycin, with acetylene-based side chains, have the highest recorded in vitro inhibitory activity against the recombinant Mycobacterium tuberculosis β-ketoacyl-ACP synthase mtFabH condensing enzyme. In particular, 5-[3-(4-acetyl-phenyl)-prop-2-ynyl]-4-hydroxy-3,5-dimethyl-5H-thiophen-2-one exhibited more than an 18-fold increased potency, compared to thiolactomycin, against this key condensing enzyme, involved in M. tuberculosis mycolic acid biosynthesis. Analogues of the antibiotic thiolactomycin, with acetylene-based side chains, have the highest recorded activity against cloned mtFabH condensing enzyme.

Gemfibrozil, a hypolipidemic drug mainly used in the treatment of hypertriglyceridemic states, strongly inhibits the rat hepatic microsomal fatty acid chain elongation system in vitro. The inhibition is independent on the reducing cofactor used in the assay. Furthermore, gemfibrozil seems to act by inhibiting the rate-limiting step of the elongation process, the condensing reaction, without discriminating among the proposed three different condensing enzymes, devoted to condensation of saturated, mono-unsaturated and polyunsaturated acyl-CoA substrates.

In chronic experimental and human pyelonephritis (PN) renal enzyme and phosphatide analyses and quantitative histological examinations were performed. The results reported in this study only refer to citrate-synthase activities (= condensing enzyme = CE). 139 rabbits developed unilateral experimental pyelonephritis, in further 19 rabbits the experimental PN did not settle or "healed up spontaneously". 31 samples of human pyelonephritic nephrocirrhotic kidneys and 20 samples of healthy human kidneys were examined in the same manner. The glomerular CE-activities in the rabbit increased steeply in the 20- and 31-days-series, remained at a high level up to the 100-days-series, showed normal values in the 213-days-series, and permitted a marked decrease to be seen in the 261-days-series only. The corticotubular CE-activities increased steeply in the 20- and 31-day-series, still being above the normal values in the 64-days-series. Thereafter, only reduced and strongly reduced values were observed. In the pyelonephritic medulla of rabbits the CE-activities increased very steeply during the 20-days-series, were still above the level of the normal values up to the 100-days-series, dropping then to pathologically reduced data in the 212-days-series. Thus the CE-activities showed a similar type of behaviour to that of the corresponding glomeruli. Chronically pyelonephritic contracted renal tissue in human beings also showed major reductions in CE-activities in all three tissue fractions studies. The significance calculations (universally applied t-test) showed an overwhelming majority of significant values (p less than 0.001).

By studying the intracellular intermediates of acetate oxidation in Escherichia coli, complete equilibration between respiratory CO2 and carboxyl carbon of both citrate and succinate has been demonstrated. Dried preparations incorporate significant amounts of acetate carbon into all Krebs cycle intermediates. These data and those of Krampitz and co-workers, plus the finding that this organism contains a potent isocitric dehydrogenase and an aconitase, strongly suggest that E. coli does indeed respire via the tricarboxylic acid cycle. The failure to date to demonstrate an acetate-to-acetate condensing enzyme in cell-free extracts leaves much doubt whether this bacterium oxidizes acetate via an abridged type of cyclic mechanism, as the results with nonproliferating, resting cells previously suggested.

Acyl-malonyl acyl carrier protein (ACP)-condensing enzyme, prepared from extracts of Escherichia coli, catalyzes the condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP. It also catalyzes the condensation of various longer chain saturated acyl-ACP derivatives with malonyl-ACP to form the β-ketoacyl-ACP of the longer homologues. The enzyme is specific for the acyl-ACP derivatives and does not act on acyl coenzyme A derivatives (acetyl-CoA or malonyl-CoA). The enzyme has a functional —SH group and can be readily inhibited by —SH-binding reagents such as N-ethylmaleimide and iodoacetamide. Acetyl-ACP protects the enzyme against thiol-binding reagents, which indicates that an acetyl—S—enzyme complex may be an intermediate in the condensation of acyl-ACP and malonyl-ACP to form β-ketoacyl-ACP.

Increased levels of condensing enzyme are shown to occur in the sera of dogs after experimental myocardial infarction. The animals with the elevated levels also showed characteristic changes of the electrical activity of the heart and associated increases in the glutamate-oxal acetate transaminase activity of serum. Condensing enzyme has been detected in the sera of patients who suffered myocardial infarctions.

The effect of tumbling trauma on the in vivo aerobic carbohydrate metabolism of normal and traumatized rats has been investigated through the ability of individual organs to convert acetate-2-C14 to nonvolatile metabolites. The ability of the brain and heart to metabolize the acetate presented to their organs is not impaired. However, the ability of the liver to do this is very markedly impaired, the muscle less so and the kidney intermediate between the latter two. By following the changes in the specific activity, it has been shown that the ability of these organs to convert citrate to succinate to malate is unaffected. It is postulated that the block in the aerobic cycle may be in either the condensing enzyme or the acetyl-Co-A kinase.