PRPP synthetase from rat liver exists as large molecular weight aggregates composed of at least four different components, i.e., two isoforms of 34 kDa catalytic subunits, PRSI and PRSII, and two associated proteins of 39 kDa and 41 kDa (PAP39 and PAP41). The four proteins have remarkably similar amino acid sequences and form a relatively large group of PRS family. PAPs, however, have extra regions of 40 to 50 amino acid residues, totally dissimilar to sequences of the catalytic subunits and thus form a subfamily. PAPs suppress the catalytic activity of PRS. They are very likely to be enzymatically inactive. The relative amounts of the mRNAs of the four components vary with the tissues, hence composition and properties of PRPP synthetase would also differ. Future studies on the physiology and pathology of PAPs are critical in elucidation of pathogenesis of some types of hyperuricemia.