The Sulfur Chemistry of Shiitake Mushroom

Allium herbs, such as Chinese chive, garlic, and onion, share a common sulfur biochemistry that occurs on cell breakage. Sulfoxide precursors are converted enzymatically to sulfenic acid intermediates and thence to a variety of pungent and in some cases noxious sulfur species that probably act to deter herbivores. Very similar biochemistry has been proposed to occur in shiitake mushrooms. Prior to the present work, our understanding of the sulfur biochemistry of these plants and fungi has been derived largely from conventional analysis procedures. We have used in situ sulfur K-edge X-ray absorption spectroscopy in intact and disrupted allium plants and shiitake mushroom. The expected changes in sulfur forms following cell breakage are indeed observed for the alliums, but no significant changes occur for the fungus. Thus, any changes involving the sulfur-containing compounds of shiitake mushroom following cell breakage occur to a far smaller extent than those involving allium plants, presumably reflecting the need in shiitake for action by multiple enzymes, namely a γ-glutamyl transpeptidase and a C−S lyase. The shiitake C−S lyase occurs in far lower concentrations than the corresponding enzyme in garlic. Furthermore, cleavage of the flavorant precursor by the shiitake C−S lyase is reported to cease before cleavage of the precursor has been completed, presumably due to a product or suicide inhibition mechanism.