Membrane-bound thioesterase activity in mycoplasmas

Abstract

Thioesterase activity was found in all mycoplasmas tested. Activity was highest in Acholeplasma species, whereas most of the sterol-requiring Mycoplasma species showed little activity. The thioesterase activity of Acholoplasma laidlawii is confined to the cell membrane. The enzyme could not be released from the membrane by either low- or high-ionic-strength solutions, with or without ethylenediaminetetraacetic acid, nor solubilized by detergents. The enzyme has a general specificity for long-chain saturated and unsaturated fatty acid thioesters. The preferred substrates among the saturated fatty acyl derivatives are the myristyl and palmityl derivatives. Arrhenius plots of thioesterase activities in A. laidlawii membranes enriched with elaidic or palmitic acids showed discontinuities at 12 and 18 degrees C, respectively. The possible regulatory significance of the thioesterase activity for the fatty acid synthetase and the possibllity that the activity of the enzyme is controlled by the physical state of membrane lipids are discussed.

The mycoplasmas.

“…According to McElhaney (174), up to about one-half of the membrane lipid in A. laidlawii may be transformed to the gel state without apparent effects on cell growth, and the existence of less than one-tenth of the membrane lipid in a fluid state is sufficient to support some cell growth and replication, albeit at greatly reduced rates. Once the lipid bilayer totally crystallizes, cells stop growing and the membrane loses its elasticity, so that the cells lyse rather than swell when placed in hypotonic solutions (336); the permeability of the cells to nonelectrolytes, such as glycerol (175), and the valinomycin-induced leakage of K+ and Rb+ (335) are reduced to zero; and the activities of some membrane-associated enzymes drop THE MYCOPLASMAS 439 sharply (69,133,263,271). If these and other harmful effects are to be avoided, a mechanism for regulating membrane fluidity becomes essential.…”

“…Arrhenius plots of the thioesterase activity showed breaks at temperatures corresponding to the membrane lipid phase transition, resem-THE MYCOPLASMAS 443 bling the ATPase activity. In the case of the thioesterase, however, it is possible that the effect of the lipid phase transition on the enzyme activity is due to changes in the solubility of the hydrophobic substrate in the bulk of membrane lipids, rather than to conformational changes in the enzyme molecule itself (271). Disposition of Membrane Proteins Transbilayer distribution of proteins.…”

Mycoplasma cells stimulate in vitro activation of plasminogen by purified tissue-type plasminogen activator

“…reaction. Dicyclohexylcarbodiimide (DCCD) which inactivates mycoplasma proton-translocating F1-F0-ATPase [20], and carbonyl cyanide mchlorophenylhydrazone (CCCP) which suppresses mycoplasma transmembrane proton gradient [21], were ineffective. A slight enhancement of activity occurred with pronase-treated mycoplasmas, indicating the involvement of membrane proteasesensitive sites in plasminogen activation.…”