JBC, Vol. 250, Issue 18, 7352-7358, Sep, 1975
Interconversion of different molecular weight forms of human erythrocyte orotidylate decarboxylase
G. K. Brown, R. M. Fox and W. J. O'Sullivan
Orotidylate decarboxylase has been purified approximately 300-fold from
human erythrocytes. It was shown to exist in three molecular weight forms,
a probable monomer of molecular weight 62,000, a dimer, and a tetramer.
Conversion of the monomer to higher molecular weight forms was associated
with increased stability to thermal inactivation and was promoted by a
number of low molecular weight compounds, including orotic acid and
competitive inhibitors of the enzyme. Orotic acid phosphoribosyltransferase
co-purified with the decarboxylase but was much more susceptible to
inactivation. The partially purified orotidylate decarboxylase showed a
triphasic Lineweaver-Burk plot when examined over a wide range of substrate
concentrations. The separated molecular weight forms gave linear double
reciprocal plots with Km values corresponding to the three values obtained
with the erythrocyte enzyme preparation. The values obtained were 25, 3,
and 0.6 muM for the monomer, dimer, and tetramer forms, respectively.
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