Ln the experiments reported here we found that enzymatic NO synthesis in th
e yeast Candida tropicalis resembles the one in animal tissues with respect
to the substrate arginine as well as its sensitivity to potential competit
ive inhibitors. Both, NO produced by the yeast's nitric oxide synthase and
NO derived from an artificial donor, suppressed the formation of pseudomyce
lia, These results suggest to make use of NO as a tool in elucidating the m
echanism controlling mycelia generation in this yeast.
The apparent K-m towards oxygen of the yeast's nitric oxide synthase (about
50 mu M) was found to be high as compared to the apparent K-m value of the
yeast's respiratory chain (about 170 nM). From this observation it may be
concluded that under conditions of little oxygen supply the nitric oxide sy
nthase will unsuccessfully compete for oxygen with respiration, Therefore,
the formation of mycelia spontaneously occurring in yeast cultures grown in
sealed chambers can be attributed to a reduced internal NO level rather th
an limited respiratory activity.