Recent studies on the biodegradation phthalate esters in natural ecosystems, sewage, and laboratory cultures are reviewed. There is ample evidence to demonstrate that bacteria are major elements in the biodegradative processes and that in most situations complete oxidation of the aromatic ring occurs; much less is known about the catabolism of the alcoholic moiety, e.g., 2-ethylhexanol. Evidence is presented to support catabolic pathways in pseudomonads and micrococci that are initiated by successive hydrolyses of the diesters to give the phthalate anion. Thereafter a dioxygenase catalyzes the formation of 4,5-dihydro-4,5-dihydroxyphthalate, which is oxidized by an NAD-dependent dehydrogenase to give 4,5-dihydroxyphthalate, Protocatechuate, formed by decarboxylation of 4,5-dihydroxyphthalate, is the substrate for ring cleavage enzymes. Whereas flurorescent pseudomonads use the beta-ketoadipate pathway, the nonfluorescent strains and micrococci examined use of meta-cleavage (4,5-) route. All the intermediates proposed have been accumulated by enzymes purified from Pseudomonas fluorescens. Isophthalate and terephthalate (anions) are readily used as carbon sources by aerobic bacteria, and preliminary evidence is consistent with catabolic routes for these isomers converging at the ring-cleavage substrate protocatechuate. Some possible effects and interactions of synthetic organic chemicals with the natural microflora, and the influence of other vectors, is discussed in relation to the maintenance of the carbon cycle and environmental pollution.
The flow of current associated with depolarizations of the giant axon of Loligo has been described in two previous papers (Hodgkin, Huxley & Katz, 1952; Hodgkin & Huxley, 1952). These experiments were concerned with the effect of sudden displacements of the membrane potential from its resting level (V =0) to a new level (V = Vj). This paper describes the converse situation in which the membrane potential is suddenly restored from V = V1 to V = 0. It also deals with certain aspects of the more general case in which V is changed suddenly from V1 to a new value V2. The experiments may be conveniently divided into those in which the period of depolarization is brief compared to the time scale of the nerve and those in which it is relatively long. The first group is largely concerned with movements of sodium ions and the second with movements of potassium ions. METHODS The apparatus and method were similar to those described by Hodgkin et al. (1952). The only new technique employed was that on some occasions two pulses, beginning at the same moment but lasting for different times, were applied to the feedback amplifier in order to give a wave form of the type shown in Fig. 6. The amplitude of the shorter pulse was proportional to V1-V2, while the amplitude of the longer pulse was proportional to V,. The resulting changes in membrane potential consisted of a step of amplitude V1, during the period when the two pulses overlap, followed by a second step of amplitude V,. RESULTS Experiments with relatively brief depotarizations Discontinuities in the sodium current The effect of restoring the membrane potential after a brief period of depolarization is illustrated by Fig. 1. Record A gives the current associated with a maintained depolarization of 41 mV. As in previous experiments, this consisted of a wave of inward current followed by a maintained phase of
Pseudomonas fluorescens PHK uses 4,5-dihydroxyphthalate as the sole carbon source for o-phthalate catabolism. This intermediate is the substrate for a decarboxylase of the pathway yielding protocatechuate. The decarboxylase was purified to homogeneity by an affinity chromatography procedure in which the reaction product, protocatechuate, was used as a ligand. We describe some properties of the enzyme, including its apparent molecular weight of 420,000 as determined by gel filtration and of 66,000 after sodium dodecyl sulfate-polyacrylamide disc gel electrophoresis, consistent with a hexameric functional protein. The apparent Km for the substrate 4,5-dihydroxyphthalate was 10.4 ,uM. The characteristics of this enzyme are compared with those described for the isofunctional enzyme from P. testosteroni.
Phenol biodegradation was studied in batch experiments using an acclimated inoculum and initial phenol concentrations ranging from 0.1 to 1.3 g/L. Phenol depletion and associated microbial growth were monitored over time to provide information that was used to estimate the kinetics of phenol biodegradation. Phenol inhibited biodegradation at high concentrations, and a generalized substrate inhibition model based on statistical thermodynamics was used to describe the dynamics of microbial growth in phenol. For experimental data obtained in this study, the generalized substrate inhibition model reduced to a form that is analogous to the Andrews equation, and the biokinetic parameters max, maximum specific growth; Ks', saturation constant; and Ki', inhibition constant were estimated as 0.251 h 2 1 , 0.011 g/L, and 0.348 g/L, respectively, using a nonlinear least squares technique. Given the wide variability in substrate inhibition models used to describe phenol biodegradation, an attempt was made to justify selection of a particular model based on theoretical considerations. Phenol biodegradation data from nine previously published studies were used in the generalized substrate inhibition model to determine the appropriate form of the substrate inhibition model. In all nine cases, the generalized substrate inhibition model reduced to a form analogous to the Andrews equation suggesting the suitability of the Andrews equation to describe phenol biodegradation data.
A Gram‐positive bacterium with the ability to utilize terephthalic acid as sole carbon source was isolated from soil. The strain was identified as a Bacillus sp. Protocatechuate was shown to be a key intermediate in the degradation of terephthalate. Oxygen uptake studies were carried out with the probable intermediates. The presence of different enzymes was tested for. A mechanism is proposed for the degradation of terephthalate.
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