Molecular and channel-forming characteristics of gramicidin K's: a family of naturally occurring acylated gramicidins.

The gramicidin K family is a set of naturally occurring acylated linear peptides in which a fatty acid is esterified to the ethanolamine hydroxyl of either gramicidin A or C, and possibly also to gramicidin B (Koeppe, R. E., II, Paczkowski, J. A., & Whaley, W. L. (1985) Biochemistry 24, 2822-2826). These acylated gramicidins form membrane-spanning channels in planar lipid bilayers and therefore constitute a model system with which to study the structural and functional consequences of acylation on membrane proteins. This paper serves to characterize further the channels formed by acylated gramicidins A and C and to demonstrate that these channels are structurally equivalent to the channels formed by the standard gramicidins. We also present additional evidence for the ester linkage in the natural acylated gramicidins A and C and identify the fatty acyl chains.     

Naturally acquired Pasteurella multocida subsp. multocida infection in a closed colony of rabbits: characteristics of isolates

Twelve litters, comprising 41 rabbits aged 35 to 60 days old, in a closed university colony, were monitored for acquisition of nasal Pasteurella multocida subsp. multocida infection. Isolates from 11 infected rabbits were characterized by colonial morphology, capsular type, biotype and antibiotic resistance. Selected isolates were further characterized by somatic antigen typing. Two major strains of P. multocida subsp. multocida were detected in the colony. One strain had mucoid colonies, fermented few carbohydrates and was serotype A:5, whereas, the other strain had smooth iridescent colonies, non-typeable capsular antigen, type 3 somatic antigen and fermented more than twice as many carbohydrates.     

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in rodent liver

Doses of chemicals which induce hepatocellular necrosis usually induce hepatic tumours if the dosing is frequent and is maintained for long periods. Such necrosis is usually evident within 48 h of the first administration. Similarly, chemicals that lead to marked proliferation of peroxisomes in the liver also usually induce hepatic tumours on pretracked regular dosing. For both of these phenomena failure to produce a certain level of effect, or to maintain it for sufficiently long periods, can result in the observation of a non-carcinogenic response. The exact dose/time requirements for carcinogenicity have not been defined and may be species/strain/sex-specific. Some chemicals induce liver enlargement and mitogenesis in the absence of overt hepatotoxic effects. The early phases of hepatomegaly are associated with mitogenic effects that can be measured as cells in S-phase within the first few days of administration. The later stages of hepatomegaly appear to be associated more with cellular hypertrophy. Both effects appear to be threshold-related. Further, sustained hepatomegaly is associated with proliferation of SER and the induction of a range of liver enzymes. These changes (mitogenesis, hepatomegaly, enzyme induction), in isolation, are less definitive indicators of carcinogenicity, but they occur for a sufficient number of liver-specific carcinogens that their role as early indicators is worthy of confirmed study. The major area of study required for all possible early markers of hepatocarcinogenicity is to establish the dose and time dependence of these changes in relation to the eventual appearance of tumours. Finally, the specificity of all these markers require evaluation by the study of appropriate non-carcinogens.     

Variations among cell lines in the synthesis of sphingolipids in de novo and recycling pathways

There are several pathways for the incorporation of sugars into glycosphingolipids (GSL). Sugars can be added to ceramide that contains sphinganine (dihydrosphingosine) synthesized de novo (pathway 1), to ceramide synthesized from sphingoid bases produced by hydrolysis of sphingolipids (pathway 2), and into GSL recycling from the endosomal pathway through the Golgi (pathway 3). We reported previously the surprising observation that SW13 cells, a human adrenal carcinoma cell line, synthesize most of their GSL in pathway 2. We now present data on the synthesis of GSL in four additional cell lines. Approximately 90% of sugar incorporation took place in pathway 2, and 10% or less in pathway 1, in human foreskin fibroblasts and NB41A3 neuroblastoma cells. In contrast, approximately 50-90% of sugar incorporation took place in pathway 1 in C2C12 myoblasts. The C2C12 cells divide more rapidly and synthesize 10-14 times as much GSL as the other three cell lines. In C6 glioma cells, approximately 30% of sugar incorporation occurred in pathway 1 and 60% in pathway 2. There was no relation between the utilization of pathways for GSL and sphingomyelin synthesis in foreskin fibroblasts and C2C12 cells. In both cells pathways 1 and 2 each accounted for 50% of incorporation of choline into sphingomyelin. In five of the six cell lines that we have studied, most GSL synthesis takes place in pathway 2. We suggest that when the need for synthesis is relatively low, as in slowly dividing cells, GSL are synthesized predominantly from sphingoid bases salvaged from the hydrolytic pathway. When cells are dividing more rapidly, the need for increased synthesis is met by upregulating the de novo pathway.