Biodehalogenation.

Haloorganic biocides are widely employed as soil fumigants to combat the destructive action of plant parasitic nematodes and fungi. These substances are dehalogenated by soil organisms, principally species of Pseudomonas and Flavobacteria, to nontoxic metabolities. The paths of metabolism of a vareity of simply alkyl halides are described with emphasis upon the biodehalogenation step.

Teratogens, carcinogens Terrible awful beasts But 0' the soil buggies Eat them all at feasts Simple alkyl halides are widely employed throughout California and elsewhere as soil fumigants to combat the destructive action of plant parasitic nematodes and fungi. Typical of this class of compounds are: methyl bromide, chloropicrin, ethylene dibromide, 1,2-dibromo-3-chloropropane, and cis-and trans-1,3-dichloropropene. In addition to their broad biocidal capacities, these compounds are volatile and possess good diffusion characteristics.
Analyses, in our laboratory, of the edible portion of crops grown in orchards and fields throughout the state that had been fumigated with these substances showed no organic halide to be present. On the other hand, an increase in inorganic bromide or chloride compared to that of untreated check crops could be detected. These results corroborated earlier industrial findings.
The lack of organic halide in any crop suggested that these substances may be metabolized or simply chemically degraded within the soil environment. Such is, indeed, the case, and I wish to present here some of the variety of biodehalogenation processes we have encountered. In many ways they resemble the transformations some of you have noted or expect in liver microsomes. Finally, I wish to comment on the reaction of iron porphyrins and hemeproteins with alkyl halides. This is a process we believe is fundamentally germane to the interaction of organic halides with living systems.
The hydrolysis of allyl chloride does not require the presence of bacteria but the conversion of allyl alcohol does.
Ethylene Dibromide. A study of the intoxication of the nematode Aphelenchus avenae by ethylene dibromide reveals the presence of two early metabolites just preceding death (5). The nematode represents the only organism we have found in which both a reductive dehalogenation and an apparent substitution process ensue [Eq. (9)], though the latter predominates. O-Acetylserine is the major product along with ethylene. In addition, N-acetylserine can be detected in the product mixture but it is the result of a rearrangement of the O-acetyl isomer [Eq. (10)] that occurs upon chromatography.
The dehalogenation does not occur in sterile soil under identical conditions. Ethylene Dibromide. Similarly, in about two months, ethylene dibromide is converted almost completely and quantitatively (4)  This transformation is also observed with the nematode Aphelenchus avenae (5). It is one of the initial reactions with the live nematode preceding death.

Epoxide Formation and Transhalogenation
In addition to the hydrolytic epoxide opening effected by these organisms, they are also capable of opening the oxirane with halide ion.
Both the epoxide opening and closing reactions are stereospecific trans processes. Again, because one expects to encounter epoxide intermediates in studies of vinylic halides, these reactions take on added significance.
Cisand Trans-1,3-Dichloropropene OH Of the potential substrates noted above as nematicides, only one is a vinylic halide. The first transformation of the dihalopropenes (9, 10) in soil (15) .s a nonbiological chemical hydrolysis.
These reactions could also be effected by partially purified enzymes isolated from the bacteria.
Cl -OH + Cl-(22) The rates here are only slightly faster than they are in aqueous buffer. On the other hand, the toxic chloroallyl alcohols resulting from the initial hydrolysis are nematicidal and they are readily metabolized to carbon dioxide water and chloride ion by resting cells of Pseudomonas sp. The sequence [Eq. (23)] is very similar to that observed for the metabolism of 3-bromopropanol by a different Pseudomonas (see above). The first step is rate-limiting and produces a free radical. The second step, the scavenging of the radical by the heme, is "exceedingly fast." These reactions have also been generally observed to occur with hemeproteins (13,14). The reactivity encountered fits well with that expected by simple theory (15). If the radical generated in the first cleavage bears a neighboring halogen, the olefin is generated

Oxidation of Hemeproteins by Alkyl Halides
Iron porphyrins are readily oxidized by a variety of alkyl halides at room temperature. The mechanism for the reductive dehalogenation or hydrogenolysis generally proceeds in two steps (11,12), as shown in Eqs. Thus, chlorotrifluoroethane, difluorochloroethylene, and trifluoroethylene are all possible initial metabolites. Of course, the free radical generated in the first step can of itself cause serious biological damage.
I wish to stop on this note and thank my colleagues, Eleanor Bartnicki and Nao Belser, for their patient and careful experimentation and their instruction. I also wish to pay tribute to the microorganism for their amazing capacity for organic synthesis with rather toxic substrates.