Envlron. Sci. Technol. 1005, 19, 374-375
CORRESPONDENCE Comment on "Development of a General Kinetic Model for Biodegradation and Its Application to Chlorophenols and Related Compounds" S I R Banerjee et al. (1)investigated the biodegradation rate of some chlorophenols and related compounds by pure as well as mixed cultures. They found that in most cases rate decreased with increasing lipophilicity (l-octanolwater partition coefficient, KO,)of the substrate. Interpretation of these results was based on a reaction mechanism where penetration of the compound into the organism is rate determining and occurs via two parallel pathways (2), i.e., either through the lipid layer (k3)or through the hydrophilic pores (k4)of the cell membrane. Banerjee et al. justified their results by assuming that k, is inversely related to KO,,while k4 does not depend on it, and concluded that lipid penetration is dominant for the more lipophilic compounds. However, it seems difficult to accept that lipid penetration should in any case be slower for the compounds with a higher lipophilicity. In addition, Banerjee et al., (1)did not consider that chlorophenols are known as inhibitors of bacterial activity (3) and in particular of the biodegradation of phenol (4),their toxicity being an increasing function of KO,. Therefore, we suggest a different interpretation, based on the idea that enzymatic processes, rather than transport phenomena, are rate determining. In fact, according to the usual kinetic treatment, when chlorophenols act as inhibitors of the phenol degradation, they tend to bind to the enzymes, giving dead-end or slowly reacting complexes. An analogous behavior could be shown when they are subjected to biodegradation. If, along a series where KO, increases, there is (i) an increasing tendency to form complexes with enzymes and (ii) a decreasing reactivity of such complexes, both the inhibition (4)and the degradation ( I ) results can be explained. The properties i and ii need not be physically related to lipophilicity; they could actually depend on structural, steric, or electronic factors that are correlated to KO,. Without aiming at a detailed description of the biodegradation process, a simplified model can be presented to support our interpretation. The degradation of a substrate S in the presence of an enzyme E and an inhibitor I may follow a rate law of the form
where C's are concentrations and ks refers to the reactivity of the complex ES and K s to its stability, while KI refers to the stability of enzymatic complexes including the inhibitor. On the other hand, inhibitor I, if similar to s,may be slowly degraded by the same enzyme E; a rate law such as eq 2 can be written for experiments in the absence of
kICICEO rI = (2) KI + CI substrate S, where where kI refers to the reactivity of the complex E1 and KI to its stability, as above. Now let us consider a series of inhibitors of increasing 374
Environ. Sci. Technol., Vol. 19, No. 4, 1985
complexation ability (Le., decreasing KI) and decreasing reactivity of the relevant complexes (decreasingkI). From eq 1 it comes out that rs decreases, independently of the reaction order; because of eq 2, rI is subjected to contradictory influences, but in cases of zero-order processes, i.e., when KI is negligible in the denominator of eq 2, rI certainly decreases. These facts, with phenol as S and the chlorophenols as compounds I, have actually been observed (1, 4 ) .
It remains to be clarified why enzymatic parameters KI and kI should be correlated to KO,values of compounds I. In our opinion, this correlation is a mathematical fact that could mask some different phenomenon, involving physical properties other than lipophilicity. The latter, for instance, is known to increase with molecular size, when aprotic substituents are progressively added into a reference compound: molar refractivity, which is strongly dependent on molecular size, was found significantly correlated to log KO,( r = 0.73) for a series of chloro- and nitrophenols (4). Within a class of compounds differing only for number and positions of identical substituents, also electronic factors may be correlated to the lipophilicity index: for the 19 chlorophenols, log KO,and Hammett x u values (4) are linearly related with a correlation coefficient r = 0.93. It should be noted that the model of Banerjee et al. (1) did not fit the behavior of some compounds, in particular, two 2,6-chlorine-substitutedphenols, for which a different rate-determining step was required. With the present interpretation, these chlorophenols are not exception, but only compounds with lower KI and kI values than their isomers, if any, with at least one ortho position free.
Literature Cited (1) Banerjee, S.; Howard, P. H.; Rosenberg, A. M.; Dombrowski, A. E.; Sikka, H.; Tullis, D. L. Enuiron. Sci. Technol. 1984, 18, 416-422. ( 2 ) Yonezawa, Y.; Urushigawa, Y. Chemosphere 1979, 8, 139-142. (3) Liu, D.; Thompson, K.; Kaiser, K. L. E. Bull. Enuiron. Contam. Toxicol. 1982,29, 130-136. ( 4 ) Beltrame, P.; Beltrame, P. L.; Carniti, P. Chemosphere 1984, 13, 3-9.
P. Beltrame," P. L. Beltrame, P. Carnltl
Departimento Chimica Fisica e d Elettrochimica Universitl di Milano 20133 Milano, Italy
SIR: Beltrame et al. (I) postulate that the biodegradability of the various compounds considered in the title study (2) is governed by the toxicity of the materials to the organism. They base their proposal on their eq 2 ( I ) , which is, in effect, the familiar Michaelis-Menten equation (eq 1)which has no explicit link to toxicity. They contend (1) u = Vmax[Sl/(Km + [SI) that under conditions where [SI > K,, the observed is nominally related to lipophilicity. zero-order rate, V, In our study, the first-order rate data of Yonezawa and Urushigawa (34) were also interpreted through our model
0013-936X/85/0919-0374$01.50/0
0 1985 American Chemical Society
