Expression of rat phosphoribosylpyrophosphate synthetase subunits I and II in Escherichia coli. Isolation and characterization of the recombinant isoforms.

The 34-kDa subunit of rat liver phosphoribosylpyrophosphate synthetase is a mixture of the two highly homologous isoforms, PRS I and PRS II. Heretofore, it was not possible to separate the two. We now describe isolation and characterization of the recombinant isoforms, named rPRS I and rPRS II. The respective rat cDNAs were inserted into vectors constructed from pKK233-2 by replacing its replication origin with that of pGEM-1 and expressed in Escherichia coli. The rPRS I and rPRS II were purified to apparent homogeneity with specific activities of 33,400 and 46,200 milliunits/mg, respectively; these values were at least 2.5-fold higher than the highest value for the mammalian enzyme so far reported. Both isoforms showed a similar dependency on Pi as an absolute activator. Sulfate partially substituted for Pi. The maximal activities of rPRS I and rPRS II with sulfate were 43 and 7%, respectively, of those seen with Pi. The two isoforms differed in sensitivity to inhibition by ADP and GDP. Inhibition of rPRS I and rPRS II by 0.3 mM ADP was 87 and 54%, respectively, and inhibition by 1 mM GDP was 93 and 24%, respectively. rPRS II was 180-fold more sensitive than rPRS I to heat inactivation at 49 degrees C.