KINETIC AND REGULATORY PROPERTIES OF RAT-LIVER PHOSPHORIBOSYLPYROPHOSPHATE SYNTHETASE COMPLEX ARE PARTLY DISTINCT FROM THOSE OF ISOLATED RECOMBINANT COMPONENT CATALYTIC SUBUNITS

Rat liver phosphoribosylpyrophosphate (PRPP) synthetase exists as comp lex aggregates composed of two catalytic subunits (PRS I and II, in a ratio of approximately 4:1) and two catalytically inactive PRPP synthe tase-associated proteins. To better understand the significance of the complex structure, the properties of the native liver enzyme were com pared with those of homologous aggregates of recombinant PRS I and PRS II (rPRS I and rPRS II), (1) The specific activity per catalytic subu nits of the liver enzyme was about 2.5 times lower than that of rPRS I over a wide pH range, K-m values for substrates and K-a values for P- i and Mg2+ of the three enzymes were similar, (2) Specific activity of the liver enzyme for the reverse reaction was about 2 times lower tha n those of rPRSs, K-m values for substrates of the three enzymes were comparable, (3) The liver enzyme was more stable than were rPRSs when incubated at a high temperature or in the absence of stabilizing agent s, (4) The liver enzyme was markedly less sensitive to inhibition by n ucleotides compared to rPRS I, GDP at 1 mM inhibited the liver enzyme and rPRS I by 32 and 93%, respectively, This effect is not ascribable to molecular interaction between rPRS I and II, as reconstitution of t he two did not alter the sensitivity to nucleotide inhibition, (5) Our observations suggest that complex aggregation states of the native en zyme not only suppress the activities but also stabilize the catalytic subunits and the associated proteins and remarkably reduce the sensit ivity to inhibition by nucleotides.