Isotope labelling experiments on the formation pathway of 3‐hydroxy‐4,5‐dimethyl‐2(5 H )‐furanone from l ‐isoleucine in cultures of Laetiporus sulphureus

Sotolon (3‐hydroxy‐4,5‐dimethyl‐2(5 H )‐furanone) imparts a strong seasoning odour reminiscent of meat broth. Previous work indicated a formation via l ‐4‐hydroxyisoleucine in surface cultures of the basidiomycete Laetiporus sulphureus grown on wheat gluten, but an enzymatic hydroxylation of l ‐isoleucine was not proven. 13 C‐Labelled l ‐leucine, l ‐isoleucine and 18 O‐water were used to elucidate the metabolism of the amino acids and proved the existence of an oxygenase activity in the pathway to sotolon. l ‐Isoleucine was converted into the corresponding α ‐keto acid and then oxygenated in the C4‐position to give 4‐hydroxy‐3‐methyl‐2‐oxopentanoic acid, the direct precursor of sotolon. A similar biotransformation was not observed for l ‐leucine. The results provide insight into the cold formation of a potent flavour compound previously associated with the thermal processing of food. Copyright © 2014 John Wiley & Sons, Ltd.