Lipoquinones of some bacteria and mycoplasmas, with considerations on their functional significance

Abstract

In a comparative study the lipoquinones of some chemoorganotrophic, facultatively aerobic bacteria, and representative Acholeplasma, Mycoplasma, Spiroplasma, and Thermoplasma strains were investigated. The quinones were partly purified by preparative thin layer chromatography of lipid extracts, and characterized by their difference spectra (reduced minus oxidized) and R_f values. Respiring bacteria expectedly contained benzoquinones and/or naphthoquinones in micromolar concentrations whereas some aerotolerant, cytochrome-less, gram-positive bacteria were found to contain menaquinones in nanomolar concentrations, or even no quinones; only Streptococcus faecalis, an organism supposed to use a rudimentary, flavin-terminated respiratory chain system produced desmethyl menaquinone in amounts ranging between “high” and “low” quinone contents. Among the mycoplasmas investigated, only Thermoplasma acidophilum was found to be capable of synthesizing quinones (MK-7) in the micromolar order of magnitude indicating a respiratory electron transport system. The presence of energetically useful respiratory chain systems in Acholeplasma, Mycoplasma, and Spiroplasma is questioned since these organisms contain quinones (MK-4) in nanomolar concentrations, or no quinones, depending on the presence of exogeneous MK-6 in the growth medium. The possible metabolic role of menaquinones present in “low” amounts, as well as the role of NADH oxidase systems more or less tightly bound to the cytoplasmic membrane with the mycoplasmas deserves further investigation.