INHIBITION OF DELTA-5-DESATURASE IN POLYUNSATURATED FATTY-ACID BIOSYNTHESIS BY (-)-ASARININ AND (-)-EPIASARININ

An extract of Chinese medicine, saishin (Asiasari radix), was found to increase the mycelial dihomo-gamma-linolenic acid content of an arachidonic acid-producing fungus, Mortierella alpina, with an accompanying decrease in its arachidonic acid content. The factors responsible for this phenomenon were isolated and identified as (-)-asarinin and (-)-epiasarinin, which are the enantiomers of (+)-episesamin and (+)-sesamin, respectively. The inhibitory effects on the DELTA-5-desaturase in rat liver microsomes of these factors were in the order of (+)-sesamin > (-)-epiasarinin > (-)-asarinin > (+)-episesamin. Kinetic analysis showed that (-)-asarinin and (-)-epiasarinin are non-competitive inhibitors, the K(i)s for rat liver DELTA-5-desaturase being 2.8 x 10(-4) and 7.1 x 10(-4) M, respectively, which are almost the same as the values of the (+)-enantiomers.