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

Sotolon (3‐hydroxy‐4,5‐dimethyl‐2(5H)‐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. ¹³C‐Labelled l‐leucine, l‐isoleucine and ¹⁸O‐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.