BIOCHEMICAL-CHARACTERIZATION OF CHLOROMETHANE EMISSION FROM THE WOOD-ROTTING FUNGUS PHELLINUS-POMACEUS

Many wood-rotting fungi, including Phellinus pomaceus, produce chlorom ethane (CH3Cl). P. pomaceus can be cultured in undisturbed glucose myc ological peptone liquid medium to produce high amounts of CH3Cl. The b iosynthesis of CH,CI is catalyzed by a methyl chloride transferase (MC T), which appears to be membrane bound. The enzyme is labile upon remo val from its natural location and upon storage at low temperature in i ts bound state. Various detergents failed to solubilize the enzyme in active form, and hence it was characterized by using a membrane fracti on. The enzyme had a sharp pH optimum between 7 and 7.2. Its apparent K-m for Cl- (ca. 300 mM) was much higher than that for I- (250 mu M) o r Br- (11 mM), A comparison of these K-m values to the relative in viv o methylation rates for different halides suggests that the real K-m f or Cl- may be much lower, but the calculated value is high because the CH,CI produced is used immediately in a coupled reaction. Among vario us methyl donors tested, S-adenosyl-L-methionine (SAM) was the only on e that supported significant methylation by MCT. The reaction was inhi bited by S-adenosyl-L-homocysteine, an inhibitor of SAM-dependent meth ylation, suggesting that SAM is the natural methyl donor. These findin gs advance our comprehension of a poorly understood metabolic sector a t the origin of biogenic emissions of halomethanes, which play an impo rtant role in atmospheric chemistry.