Entamoeba histolytica: I. Aerobic metabolism

Abstract

The respiration of intact trophozoites harvested from axenic cultures of Entamoeba histolytica was studied with the polarographic technique utilizing the Clark oxygen electrode. A typical Q o 2 value for the freshly harvested amebae was 1 μatom oxygen/mg protein/hr. It was conclusively demonstrated that this parasite, a putative anaerobe, not only consumes oxygen when provided, but has a high affinity for the gas. Added glucose, galactose, and ethanol increased the respiratory rates, whereas other carbohydrates were without effect on the endogenous respiration. Intermediates of the tricarboxylic acid cycle, amino and fatty acids did not stimulate the respiration of E. histolytica. Inhibitors of the mammalian respiratory chain (cyanide, antimycin) as well as agents that inhibit enzymes catalyzing the tricarboxylic acid cycle (malonate, fluoropyruvate, fluoroacetate, fluorocitrate) had little effect on the endogenous or glucose-supported respiration. Alkylating agents (iodoacetamide, iodoacetate), cinnamate, and N-ethylymaleimide strongly inhibited the oxygen consumption of E. histolytica. The chemotherapeutic agents, Paromomycin, Emetine and Metronidazole, at concentrations that inhibit growth in vitro, did not restrict the respiration. Storage of the trophozoites at 4 C led to progressive deterioraion of the parasites and loss of endogenous and glucose-supported respiration. The deterioration was paralled by loss of SH-materials from the amebae. Likewise, sonication or lysis with detergents abolished both the endogenous respiration and response to glucose. Exogenous NADH or NADPH evoked only marginal increases in oxygen consumption of the freshly harvested amebae, but were effective respiratory substrates with stored or sonicated organisms. Addition of vitamin K 3 greatly enhanced the endogenous and glucose-supported respiration of the intact amebae, as well as enhancing the response of stored or sonicated amebae to reduced pyridine nucleotides.