Beschreibung:
<jats:p>Two hydroxylation reactions of <jats:italic>m</jats:italic>‐cresol, an intermediate in the secondary biosynthesis of the antibiotic patulin, have been characterised in cell‐free preparations of <jats:italic>Penicillium patulum.</jats:italic> The ability to convert <jats:italic>m</jats:italic>‐cresol to 2,5‐dihydroxytoluene and <jats:italic>m</jats:italic>‐hydroxybenzyl alcohol is located in separable fractions of the 105000 × g microsomal pellet. The activities had <jats:italic>K</jats:italic><jats:sub>m</jats:sub> values for <jats:italic>m</jats:italic>‐cresol of 80 μM and 5 μM, respectively. Both reactions require molecular oxygen and NADPH for activity. Inhibition of <jats:italic>m</jats:italic>‐cresol hydroxylases by carbon monoxide is reversible by light with a photochemical action spectrum maximal at 450 nm. Carbon monoxide difference spectra of mycel extracts show the presence of absorbing pigments at 422 and 450–455 nm.</jats:p><jats:p>The enzymes are not appreciably inhibited by cyanide but are inhibited by cytochrome <jats:italic>c. m</jats:italic>‐Cresol hydroxylation therefore appears to be catalysed by typical mixed‐function oxidases involving an NADPH‐dependent reductase and a pigment like cytochrome P‐450.</jats:p><jats:p>In the light of studies on the utilisation of radioactively labelled intermediates, the methyl hydroxylation of <jats:italic>m</jats:italic>‐cresol to <jats:italic>m</jats:italic>‐hydroxybenzyl alcohol probably represents an important reaction in the sequence leading to patulin formation. Ring hydroxylation of <jats:italic>m</jats:italic>‐cresol to 2,5‐dihydroxytoluene appears to be a side reaction functional at higher concentrations of <jats:italic>m</jats:italic>‐cresol.</jats:p>