Saccharomyces cerevisiae contains a cytosolic pathway
that involves the enzymes pyruvate carboxylase, malate
dehydrogenase (MDH) and fumarase. This pathway
enables the conversion of pyruvic acid through oxalo-
acetic acid to L-malic acid by the incorporation of CO2.
The results that directly show that L-malic acid is ac-
cumulated by the cytosolic pathway in S. cerevisiae are
as follows. (a) A
13
C nuclear magnetic resonance study
following the label from glucose to L-malic acid indicates
that L-malic acid is synthesized from pyruvic acid via
oxaloacetic acid (Pines et al. 1996) and (b) cytosolic
pathway enzymes at high activities are found in the yeast
cytosol (Haarasilta and Taskinen 1977; Minard and
McAlister-Henn 1994; Van Urk et al. 1989). In this re-
gard, the cytosolic location of pyruvate carboxylase in
yeast is the same as in the organic-acid-producing fungi
Aspergillus ¯avus and Rhizopus oryzae, and is in contrast
to its mitochondrial location in higher eukaryotes such
as mammals (Bercovitz et al. 1990; Haarasilta and
Taskinen 1977; Osmani and Scrutton 1983, 1985; Van
Urk et al. 1989). Furthermore, inhibitors of pyruvate
carboxylase, such as avidin, inhibit the formation of
L-malic acid (Schwartz and Radler 1988) while condi-
tions that favor the availability of dissolved CO2 (the
substrate of pyruvate carboxylase), stimulate L-malic
acid production (Schwartz and Radler 1988).