UDPG-pyrophosphorylase (EC 2.7.7.9) from Saccharomyces cerevisiae was studied and the presence of
isoforms investigated. Its activity was monitored during growth of cultures in rich media containing
glucose,
galactose,
sucrose,
maltose or
glycerol as
carbon sources. The results suggest that
UDPG-pyrophosphorylase is subject to both catabolite repression and catabolite inactivation. The inactivation process seems to be complex: in order to produce maximum inactivation,
glucose and
ammonium sulfate must be added together. Addition of
glucose or
ammonium sulfate separately produced little effect upon
enzyme activity. Adsorption to and elution from a
DEAE-Sephacel column of a crude
protein extract prepared from yeast cells collected in stationary phase from a
glucose medium showed three activity peaks, which we denominated
isoform I, II, and III.
Isoform I is constitutive, it was the only form present during exponential growth on
glucose medium, and did not suffer any alteration after
glucose exhaustion, heat shock or by growing cells on
maltose. On the other hand,
isoforms II and III were shown to be repressed by
glucose, and induced by heat shock. Furthermore,
isoform II of
UDPG-pyrophosphorylase was present together with
isoform I when yeast cells were grown on
maltose. The presence of a MAL4C allele rendered
isoform II constitutive. Interestingly, a gal3 mutant strain had low
UDPG-pyrophosphorylase activity and
isoforms I and II were not expressed. These results are discussed in relation to
trehalose metabolism.