Synthesis of sterigmatocystin derivatives and their biotransformation to aflatoxins by a blocked mutant of Aspergillus parasiticus

Seven alkyl and aryl homologues of O-methylsterigmatocystin (OMST) were syn thesised and fed in separate experiments to a mutant of Aspergillus parasit icus capable of converting sterigmatocystin (ST) to aflatoxin B-1 (AFB(1)). Their conversion to AFB(1) was followed over a time period and it was foun d that O-propylsterigmatocystin (OPRST) was converted to AFB(1) more rapidl y than O-ethylsterigmatocystin (OEST) or OMST or ST itself. The aryl deriva tive O-benzoylsterigmatocystin (OBzST) was converted at the slowest rate. T hese results show that alkyl and aryl homologues of OMST may be converted t o AFB(1), suggesting that the methylation of ST is not an absolute requirem ent for its conversion to AFB(1). It seems likely that whatever enzyme(s) a re involved in this process exhibit relative specificity. As to whether alk ylation of ST is an obligatory step in AFB(1) biosynthesis is neither suppo rted nor disproved as the fungal cells used are presumably capable of methy lating ST. The fact that the propyl derivative showed fastest conversion is not necessarily significant as this may be due to faster diffusion of the least polar of the derivatives through the cell membrane.