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Changes in functional activity of specific enzyme reactions in the cells of pectinolytic bacteria from the gastrointestinal tract of animals in vitro cultivated in the medium containing pectin or glucose were studied against a background of the low dose effect of the wide spread biocide pentachlorophenol alone as well as in combination with the natural sorbents clinoptilolites. Regardless of the absence of transketolase reaction in the cells of all studied strains, they metabolized highly the above substrates that are dissimilar in chemical structure and produced different products of their degradation. It has been shown that the high metabolic level in the cells is provided by the function of the unique enzymatic reaction catalyzed by 2-keto-3-desoxy-6-phosphogluconate aldolase (EC 4.1.2.14) that permits to use effectively the metabolic pathway of Entner-Doudoroff. Cells could also utilize the same substrates via the Embden-Meyerhof-Parnas pathway, therefore they possess the other key reaction that is catalyzed by fructosobiphosphate aldolase (EC 4.1.2.13). Even a low dose of PCP (20 microM) decreased sharply activity of the mentioned key enzymes and intermediates' production in the cells of the studied strains with the use of both substrates. However, presence of clinoptilolites in the medium reduced significantly the biocide inhibition effect. Furthermore, in the medium with glucose, protection of intracellular metabolism with the help of sorbents was registered more clearly than with pectin. This can evidence for more mobile and simpler possibilities of accelerated production of necessary intermediates from glucose that are capable to induce activation of the key enzymatic reactions in cells utilizing selectively the substrates (which are different in accessibility and other characteristics) under the toxic agent effect.