Metabolism of propionate to acetate in the cockroach Periplaneta americana

Structural correlation between cuticular hydrocarbons and female contact sex pheromone of German cockroachBlattella germanica (L.)

The structural relationships between the cuticular hydrocarbons and the contact sex pheromone of the female German cockroach,Blattella germanica, were investigated. Cuticular hexane extracts were separated into hydrocarbon and ketone fractions by TLC or silicic acid column chromatography. The ketone fraction (which contains the major contact sex pheromone component) was analyzed by GC-MS before and after reduction to ydrocarbon. In addition to 3,11-dimethyl-2-nonacosanone, 3,11-dimethyl-2-heptacosanone was also identified. Females have the 3,11- and 3,9-dimethyl C27 and C29 alkanes, but only the 3,11- isomer of the dimethylketones. Inddition to the hydrocarbon components previously reported, a number of new components were characterized. Although the ratios of cuticular hydrocarbons differ among nymphs, adult males, and adult females, they have qualitatively identical hydrocarbon profiles, suggesting that the production of the contact sex pheromone results from the sex-specific oxidation of 3,11-imethylalkanes to pheromone components by the female.

First biosynthetic pathway of 1-hepten-3-one in Iporangaia pustulosa (Opiliones)

Arthropods produce a great variety of natural compounds, many of which have unexplored biosynthesis. Among the armored harvestmen (Arachnida: Opiliones) of the suborder Laniatores, the defensive gland exudates contain vinyl ketones and other constituents of supposed polyketide origin. We have studied the biosynthesis of 1-hepten-3-one in the Neotropical harvestman Iporangaia pustulosa by feeding individuals with ¹³C-labeled precursors, demonstrating its mixed acetate/propionate origin. ¹³C NMR spectroscopy showed an unusual labeling pattern suggesting different propionate sources for starting and extender units. Our analysis also indicates the presence of methylmalonyl-CoA mutase, converting acetate into propionyl-CoA via succinyl-CoA, together with other C₃ unit routes. This is the first biosynthetic study of alkyl vinyl ketones in arthropods. Our results shed light on the origin and diversification of chemical compounds in a major arthropod group.

Rhodobacter sphaeroides uses a reductive route via propionyl coenzyme A to assimilate 3-hydroxypropionate

3-Hydroxypropionate is a product or intermediate of the carbon metabolism of organisms from all three domains of life. However, little is known about how carbon derived from 3-hydroxypropionate is assimilated by organisms that can utilize this C(3) compound as a carbon source. This work uses the model bacterium Rhodobacter sphaeroides to begin to elucidate how 3-hydroxypropionate can be incorporated into cell constituents. To this end, a quantitative assay for 3-hydroxypropionate was developed by using recombinant propionyl coenzyme A (propionyl-CoA) synthase from Chloroflexus aurantiacus. Using this assay, we demonstrate that R. sphaeroides can utilize 3-hydroxypropionate as the sole carbon source and energy source. We establish that acetyl-CoA is not the exclusive entry point for 3-hydroxypropionate into the central carbon metabolism and that the reductive conversion of 3-hydroxypropionate to propionyl-CoA is a necessary route for the assimilation of this molecule by R. sphaeroides. Our conclusion is based on the following findings: (i) crotonyl-CoA carboxylase/reductase, a key enzyme of the ethylmalonyl-CoA pathway for acetyl-CoA assimilation, was not essential for growth with 3-hydroxypropionate, as demonstrated by mutant analyses and enzyme activity measurements; (ii) the reductive conversion of 3-hydroxypropionate or acrylate to propionyl-CoA was detected in cell extracts of R. sphaeroides grown with 3-hydroxypropionate, and both activities were upregulated compared to the activities of succinate-grown cells; and (iii) the inactivation of acuI, encoding a candidate acrylyl-CoA reductase, resulted in a 3-hydroxypropionate-negative growth phenotype.

Sources of propionate for the biogenesis of ethyl-branched insect juvenile hormones: Role of isoleucine and valine

Corpora allata from adult female Manduca sexta biosynthesize the sesquiterpenoid juvenile hormone (JH) III and the unusual ethyl-branched homologue JH II in vitro. We maintained corpora allata in medium 199 using [methyl-(3)H]methionine as the source of the JH methyl ester moiety and as a mass marker. This allowed measurement of the relative contributions of (14)C-labeled precursors to the biogenesis of JH II and III carbon skeletons. We showed efficient incorporation of a propionate equivalent, from isoleucine or valine catabolism, into the ethyl-branched portion of JH II, using double-label liquid scintillation counting of isolated JHs and gas chromatography/mass spectrometry with selected ion monitoring of JH deuteromethoxyhydrin derivatives. Methionine was a poor source of propionate for JH II biosynthesis, while glucose, succinate, threonine, and beta-alanine did not contribute propionate at all. Leucine, isoleucine, and glucose incorporated into JH III and the acetate-derived portion of JH II.

Isolation of mutants deficient in acetyl-CoA synthetase and a possible regulator of acetate induction in Aspergillus niger

Acetate-non-utilizing mutants in Aspergillus niger were selected by resistance to 1.2% propionate in the presence of 0.1% glucose. Mutants showing normal morphology fell into two complementation groups. One class of mutant lacked acetyl-CoA synthetase but had high levels of isocitrate lyase, while the second class showed reduced levels of both acetyl-CoA synthetase and isocitrate lyase compared to the wild-type strain. By analogy with mutants selected by resistance to 1.2% propionate in Aspergillus nidulans, the properties of the mutants in A. niger suggest that the mutations are either in the structural gene for acetyl-CoA synthetase (acuA) or in a possible regulatory gene of acetate induction (acuB). A third class of mutant in a different complementation group was obtained which had abnormal morphology (yellow mycelium and few conidia); the specific lesion in these mutants has not been determined.

Propionate catabolism in Salmonella typhimurium LT2: two divergently transcribed units comprise the prp locus at 8.5 centisomes, prpR encodes a member of the sigma-54 family of activators, and the prpBCDE genes constitute an operon

We present the initial genetic and biochemical characterization of the propionate (prp) locus at 8.5 centisomes of the Salmonella typhimurium LT2 chromosome (T. A. Hammelman et al., FEMS Microbiol. Lett. 137: 233-239, 1996). In this paper, we report the nucleotide sequences of two divergently transcribed transcriptional units. One unit is comprised of the prpR gene (1,626 bp) encoding a member of the sigma-54 family of transcriptional activators; the second unit contains an operon of four genes designated prpB (888 bp), prpC (1,170 bp), prpD (1,452 bp), and prpE (1,923 bp). The heme biosynthetic gene hemB was shown by DNA sequencing to be located immediately downstream of the prpBCDE operon; hemB is divergently transcribed from prpBCDE and is separated from prpE by a 66-bp gap. In addition, we demonstrate the involvement of PrpB, PrpC, and PrpD in propionate catabolism by complementation analysis of mutants using plasmids carrying a single prp gene under the control of the arabinose-responsive P(BAD) promoter. Expression of prpB to high levels was deleterious to the growth of a prp+ strain on minimal medium supplemented with propionate as a carbon and energy source. We also report the cloning and overexpression of prpB, prpC, prpD, and prpE in the T7 system. PrpB, PrpC, PrpD, and PrpE had molecular masses of ca. 32, ca. 44, ca. 53, and ca. 70 kDa, respectively. PrpB showed homology to carboxyphosphonoenolpyruvate phosphonomutase of Streptomyces hygroscopicus and to its homolog in the carnation Dianthus caryophyllus; PrpC was homologous to both archaeal and bacterial citrate synthases; PrpD showed homology to yeast and Bacillus subtilis proteins of unknown function; PrpE showed homology to acetyl coenzyme A synthetases. We identified a sigma-54 (RpoN)-dependent promoter with a consensus RpoN binding site upstream of the initiating methionine codon of prpB, the promoter-proximal gene of the prp operon. Consistent with this finding, an rpoN prp+ mutant failed to use propionate as carbon and energy source. Finally, we report the location of MudI1734 elements inserted in prpC or prpD and of a Tn10delta16delta17 element in prpB and provide genetic evidence supporting the conclusion that the prpBCDE genes constitute an operon.

A comparative catabolism study of isoleucine by insect and mammalian tissues

The metabolism of [U-14C]isoleucine was examined in different tissues of five species of lepidopteran and four species of non-lepidopteran insects. Slices of fat body, epidermis, Malpighian tubule, gut, and muscle were incubated in a culture medium containing [U-14C]isoleucine; the medium was analyzed by ion-exclusion LC to quantify labeled metabolites. Tissues of lepidopteran insects secrete high levels of metabolites including 2-keto-4-methylvalerate, 2-methylbutyrate, propionate, and acetate. Tissues of non-lepidopteran insects secrete low amounts of these acids. Analysis of isoleucine transaminase activity in selected tissues of non-lepidopteran insects indicated that those tested contain significant activity. These results demonstrate that tissues of lepidopteran insects have a unique ability to secrete short chain acids, derived from isoleucine, into the medium. The secretion of propionate correlates with the ability to synthesize ethyl-branched juvenile hormones and indicates the presence of an efficient transport system for short chain acids. We also monitored the secretion of acidic metabolites of isoleucine by different tissues of the rat. Muscle was most active in secreting keto acid whereas heart secreted high levels of 2-methylbutyrate. Negligible quantities of metabolites of isoleucine were secreted by the liver.

Comparative aspects of propionate metabolism

1. The catabolism of propionate has been studied extensively in vertebrates and the major pathway has been shown to be its derivatization to propionyl-CoA, carboxylation to D-methylmalonyl-CoA, isomerization to L-methylmalonyl-CoA and then conversion to succinyl-CoA via a vitamin B12 dependent methylmalonyl-CoA mutase. 2. By contrast, in all insect species studied to date, many of which do not contain detectable levels of vitamin B12, the major metabolic pathway of propionate is its conversion to 3-hydroxypropionate and then to acetate. Carbon-3 of propionate becomes the carboxyl carbon of acetate and carbon-2 of propionate becomes the methyl carbon of acetate. 3. A number of species of non-insect arthropods and other invertebrates contain relatively high levels of vitamin B12 and catabolize propionate by the same pathway as that of vertebrates. Under anoxic conditions, some invertebrates, including bivalves, convert succinate to propionate. 4. In plants, evidence has been presented for the metabolism of propionate to both acetate and succinate. Micro-organisms possess a myriad of pathways by which they produce and catabolize propionate.

Forced synthesis of trace amounts of juvenile hormone II from propionate by corpora allata of a juvenile hormone III-producing insect

Corpora allata of the cockroach Diploptera punctata normally synthesize only the isoprenoid juvenile hormone III (JH III). Only under extreme in vitro conditions (absence of carbon sources other than propionate) do they produce trace amounts of the homoisoprenoid JH II in addition to JH III. The specificity of the in vitro synthesis of JH III by D. punctata is thus consistent with the observed lack of homoisoprenoid JHs in this insect.