Hypoxanthine-guanine phosphoribosyltransferase from beef brain: a trimer

Artemia purine phosphoribosyltransferases. Purification and characterization

Adenine phosphoribosyltransferase (APRTase) and hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) have been purified from Artemia cysts and nauplii to apparent homogeneity, as determined by SDS-PAGE. The purification includes affinity chromatography on AMP-Sepharose, which binds both enzymes, and they are eluted at different 5-phospho-alpha-D-ribosyl diphosphate (PP-Rib-P) concentrations. The purified enzymes from Artemia cysts were similar to nauplii enzymes with respect to Mr in denaturing gel electrophoresis and gel filtration, pH and cation dependence and kinetic constants for substrates and inhibitors. By Sephadex G-100 filtration, the native Mr of the adenine and hypoxanthine-guanine enzymes was estimated to be Mr 28,000 and 66,000, respectively. Analysis by SDS-PAGE revealed that the APRTase was a dimer of Mr 15,000 sub-units and the HGPRTase, a tetramer of four identical Mr 19,000 sub-units. The pH profile of the HGPRTase shows two apparent buffer-independent pH optima, at 7.0 and 9.5, while the APRTase has just one, at about pH 8-9. The purine phosphoribosyltransferase activity with adenine was highest, about tenfold the HGPRTase activity with hypoxanthine and fivefold that with guanine. Both enzymes exhibited similar requirements for divalent cations, either Mg2+, Mn2+ or Zn2+, while Ca2+ is highly inhibitory. The Km values of APRTase for adenine and PP-Rib-P are 2 and 30 microM, respectively, and the Km values of HGPRTase for hypoxanthine, guanine and PP-Rib-P are less than 1, less than 1 and 15 microM, respectively. Plots of the reciprocal enzyme activities versus reciprocal concentrations of one substrate at several fixed levels of the second one yield a pattern of inhibition by guanine and hypoxanthine. Product-inhibition studies indicated that AMP is a competitive inhibitor with respect to PP-Rib-P in the APRTase reaction, while the HGPRTase shows a mixed inhibition by GMP.

Evidence for tetrameric structure of mammalian hypoxanthine phosphoribosyltransferase

A fast electrophoretic variant of hypoxanthine phosphoribosyltransferase (HPRT) has been detected in Mus musculus bactrianus, a mouse subspecies from Middle Asia (USSR). The electrophoretic HPRT pattern yielded by hybrids between the somatic cell of LMTK- (deficient in thymidine kinase) and the splenocytes of a male of M. m. bactrianus was five-banded. The pattern obtained from the germ cells of the ovaries from 14.5-day-old embryos from laboratory CBA mice X M. m. bactrianus crosses was also composed of five bands. The hybrids between the somatic cells of human fibroblasts X LMTK- cells gave a three-banded electrophoretic HPRT pattern because the asymmetrical heteropolymeric isozymes are probably unstable. Taken together, all the evidence is in favor of a tetrameric structure of mammalian HPRT.