Leaves of Populus deltoides emit acetaldehyde under various conditions, and we have discovered that leaf veins and petioles, seen in this image, contain the enzyme pyruvate decarboxylase (PDC), which produces acetaldehyde as a product. We have purified leaf vein PDC 143-fold, and it appears to act as a high affinity PDC, operating under the semi-aerobic conditions occurring in vascular bundles.

A majority of heterotrophic bacteria isolated from soil, water, sediment, vegetation, and marine algae cultures methylated sulfide, producing methanethiol. This was demonstrated with intact cells by measuring the emission of methanethiol with a sulfur-selective chemiluminescence detector, and in cell extracts by detection of sulfide-dependent thiol methyltransferase activity. Extracts of two Pseudomonas isolates were fractionated by gel-filtration and ion-exchange chromatography, and with sulfide as the substrate a single peak of thiol methyltransferase activity was seen in each case. Extracts of several bacterial strains also contained thiol methyltransferase activity with organic thiols as substrates. Thus, S-adenosylmethionine-dependent thiol methyltransferase activities are widespread in bacteria and may contribute to biogenic emissions of methylated sulfur gases and to the production of methyl thioethers.

Cell extracts from the ciliate Tetrahymena thermophila catalyzed the S-adenosylmethionine-dependent methylation of sulfide. The product of the reaction, methanethiol, was detected by a radiometric assay and by a gas-chromatographic assay coupled to a sulfur-selective chemiluminescence detector. Extracts also catalyzed the methylation of selenide, and the product was shown by gas chromatography-mass spectrometry to be methaneselenol. The sulfide and selenide methyltransferase activities copurified with the aromatic thiol methyltransferase previously characterized from this organism (A.-M. Drotar and R. Fall, Pestic. Biochem. Physiol. 25:396-406, 1986), but heat inactivation experiments suggested the involvement of distinct sulfide and selenide methyltransferases. Short-term toxicity tests were carried out for sulfide, selenide, and their methylated derivatives; the monomethylated forms were somewhat more toxic than the nonmethylated or dimethylated compounds. Cell suspensions of T. thermophila exposed to sulfide, methanethiol, or their selenium analogs emitted methylated derivatives into the headspace. These results suggest that this freshwater protozoan is capable of the stepwise methylation of sulfide and selenide, leading to the release of volatile methylated sulfur or selenium gases.

The protozoan Tetrahymena thermophila produces pentachlorobenzenethiol as an intermediate during the metabolism of the fungicide pentachloronitrobenzene. A thiol methyltransferase which catalyzes the S-adenosylmethionine-dependent methylation of pentachlorobenzenethiol has been detected in extracts from this organism. The enzyme was localized in the cytoplasm, and has been purified 130-fold. It has a molecular weight of 41,000 and a pH optimum of 7.5. A number of foreign thiols, but not the cellular thiols, glutathione or cysteine, were substrates for the enzyme. Phenols and anilines were not substrates. With pentachlorobenzenethiol as the methyl acceptor the apparent Km was 27 μM, and the Km for S-adenosylmethionine was 110 μM. To determine if the enzyme could play a role in the detoxication of reactive thiols, toxicity tests were carried out with a variety of aromatic thiols and their corresponding methylthio derivatives. In addition, toxicity tests on some corresponding phenols and anilines were carried out. In general, in long-term tests the aromatic thiols were more toxic than the methylthio derivatives, and intermediate in toxicity between phenols and anilines. In short-term tests the toxicity of aromatic thiols was enhanced by pretreatment with dithiothreitol suggesting that the thiol/disulfide status is an important factor in thiol toxicity. Exposure of T. thermophila to sublethal levels of pentachlorobenzenethiol resulted in an adapted population of cells with increased levels of thiol methyltransferase, and decreased sensitivity to aromatic thiols.

Extracts from cultured plant cells of spinach, maize and sycamore and from Lemna plants contain detectable glutathione peroxidase activity, using either hydrogen peroxide or t-butyl hydroperoxide as substrates. Using extracts from cultured maize cells, two peaks of glutathione peroxidase activity could be resolved by a combination of gel filtration and ion exchange chromatography. One peak was eluted along with glutathione transferase activity; the second was distinct from both glutathione transferase and ascorbic acid peroxidase, and was active with both hydrogen peroxide and organic hydroperoxides. It seems likely that at least two enzymes with glutathione peroxidase activity exist in higher plant cells.

The green alga Euglena gracilis contains a thiol methyltransferase that catalyzes the S-adenosylmethionine-dependent methylation of pentachlorobenzenethiol. The enzyme was localized in the cytoplasm and partially purified. The pH optimum for the enzyme was 6.5. The enzyme methylated a number of foreign thiols, but not the cellular thiols, glutathione or cysteine. Phenols and anilines were not substrates. When pentachloro-benzenethiol was the methyl acceptor the Km was found to be 82 μm and the corresponding Km for S-adenosylmethionine was 140 μm. The molecular weight of the enzyme was 21,000, as determined by gel filtration. A role for this enzyme in detoxifying xenobiotic thiols is proposed.