Metabolism of phospholipids and lysophospholipids by Trypanosoma brucei.

African trypanosomes (Trypanosoma brucei brucei) rapidly metabolize exogenous 1-acyl-lysophospholipids by at least two routes: (1) hydrolysis by a phospholipase A1; (2) acylation by an acyl-CoA-dependent acyltransferase. In contrast to lysophospholipids, exogenous phospholipids are not rapidly metabolized by T. brucei. The acyltransferase (EC 2.3.1.23) converts exogenous 1-acyl lysophosphatidylcholine and exogenous acyl-CoA to phosphatidylcholine and CoA-SH. It is a membrane-bound enzyme and shows maximal activity within the first 2 min of exposure of trypanosomes to the exogenous substrates. The acyltransferase specificity for lysophospholipids is lysophosphatidylcholine greater than lysophosphatidylinositol greater than lysophosphatidylethanolamine greater than lysophosphatidate. Phosphatidylcholine enhances the enzyme activity towards lysophosphatidylethanolamine and lysophosphatidic acid. The preference for CoA acyl thioesters is oleoyl greater than palmitoyl greater than myristoyl greater than stearoyl greater than arachidonoyl, and this specificity distinguishes the protozoan enzyme from those of cells of mammalian hosts, which are specific for arachidonoyl-CoA. When the acyltransferase converts exogenous lysophosphatidylethanolamine to phosphatidylethanolamine, the latter is rapidly methylated to form dimethylphosphatidylethanolamine. There is also rapid hydrolysis of exogenous oleoyl-CoA by a thioester hydrolase in living trypanosomes, to yield free oleate and CoA-SH.

Co-mitogenic tumor promoters suppress the phosphatidylinositol response in lymphocytes during early mitogenesis.

The tumor-promoting agents 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and phorbol-12,13-dibenzoate inhibited the increased accumulation of [32P]phosphatidylinositol (PI) induced in mouse spleen lymphocytes by mitogenic lectins in the presence of [32P]orthophosphate. Similar inhibition of [32P]PI levels by TPA was seen in human tonsil T-lymphocytes stimulated with phytohemagglutinin. Only co-mitogenic phorbol esters prevented the [32P]PI accumulation during early mitogenesis. No increased 32P-labelling due to mitogen or decreases due to TPA was observed when cells were equilibrated with [32P]orthophosphate for 24 h prior to stimulation with mitogen, from which it is concluded that the total concentrations of phosphatidylcholine (PC) and PI are unaffected by mitogen or co-mitogen. The [32P]PI elevation but not the [32P]PC elevation was proportional to T-cell mitogenic potency for the lectins concanavalin A, divalent succinyl concanavalin A and phytohemagglutinin, and was prevented in each case by 5 X 10(-8) M TPA. Escherichia coli lipopolysaccharide did not give increased 32P incorporation into PI or PC, and TPA had no effect on 32P labelled phospholipid levels in the presence of this B-cell mitogen. The results indicate that the phosphatidylinositol response is not an invariable correlate of T-cell mitogenesis by polyclonal mitogens.

Two proline porters in Escherichia coli K-12.

Escherichia coli mutants defective at putP and putA lack proline transport via proline porter I and proline dehydrogenase activity, respectively. They retain a proline uptake system (proline porter II) that is induced during tryptophan-limited growth and are sensitive to the toxic L-proline analog, 3,4-dehydroproline. 3,4-Dehydroproline-resistant mutants derived from a putP putA mutant lack proline porter II. Auxotrophic derivatives derived from putP+ or putP bacteria can grow if provided with proline at low concentration (25 microM); those derived from the 3,4-dehydroproline-resistant mutants require high proline for growth (2.5 mM). We conclude that E. coli, like Salmonella typhimurium, possesses a second proline porter that is inactivated by mutations at the proP locus.