Metabolism of hypoxanthine in isolated rat hepatocytes.

The hepatic metabolism of hypoxanthine was investigated by studying both the fate of labelled hypoxanthine, added at micromolar concentrations to isolated rat hepatocyte suspensions, and the kinetic properties of purified hypoxanthine/guanine phosphoribosyltransferase from rat liver. More than 80% of hypoxanthine was oxidized towards allantoin; less than 5% of the label was incorporated into the purine mononucleotides, and a similar proportion appeared transiently in inosine. The maximal velocity of oxidation (approx. 750nmol/min per g of cells) was in close agreement with the known activity of xanthine oxidase in liver extracts. In contrast, the maximal velocity of the incorporation of labelled hypoxanthine into mononucleotides reached only 30nmol/min per g of cells, compared with an activity of hypoxanthine/guanine phosphoribosyltransferase, measured at substrate concentrations analogous to those prevailing intracellularly, of 500nmol/min per g of cells. Hypoxanthine incorporation into the mononucleotides was decreased by allopurinol, anoxia and ethanol, despite inhibition of its oxidation under these conditions; it was increased by incubation of the cells in supraphysiological concentrations of Pi. Allopurinol and anoxia decreased the concentration of phosphoribosyl pyrophosphate inside the cells by respectively 40 and 60%, ethanol had no effect on the concentration of this metabolite and Pi increased its concentration up to 10-fold. The kinetic study of purified hypoxanthine/guanine phosphoribosyltransferase showed that a mixture of ATP, IMP, GMP and GTP, at the concentrations prevailing in the liver cell, decreased the V max. of the enzyme 6-fold, increased its Km for hypoxanthine from 1 to 4 microM and its Km for phosphoribosyl pyrophosphate from 2.5 to 25 microM. In the presence of 5 microM-hypoxanthine and 2.5 microM-phosphoribosyl pyrophosphate, the mixture of nucleotides inhibited the activity of purified hypoxanthine/guanine phosphoribosyltransferase by 95%. It is concluded that this inhibition results in a limited participation of hypoxanthine/guanine phosphoribosyltransferase in the control of the production of allantoin by the liver.     

Human hypoxanthine-guanine phosphoribosyltransferase. IMP-GMP exchange stoichiometry and steady state kinetics of the reaction.

Steady state kinetics of hypoxanthine guanine phosphoribosyltransferase-catalyzed reactions are studied. The results obtained suggest that IMP, GMP, P-Rib-PP, and pyrophosphate bind to the same enzyme form, while hypoxanthine and guanine bind to a different form. Guanine activates IMP-pyrophosphorolysis. During the reaction guanine and IMP are consumed with formation of GM     

Metabolism of Guanine and Guanine Nucleotides in Primary Rat Neuronal Cultures

The metabolic fate of guanine and of guanine ribonucleotides (GuRNs) in cultured rat neurons was studied using labeled guanine. 8-Aminoguanosine (8-AGuo), an inhibitor of purine nucleoside phosphorylase, was used to clarify the pathways of GMP degradation, and mycophenolic acid, an inhibitor of IMP dehydrogenase, was used to assess the flux from IMP to GMP and, indirectly, the activity of the guanine nucleotide cycle (GMP----IMP----XMP----GMP). The main metabolic fate of guanine in the neurons was deamination to xanthine, but significant incorporation of guanine into GuRNs, at a rate of approximately 8.5-13.1% of that of the deamination, was also demonstrated. The turnover rate of GuRNs was fast (loss of 80% of the radioactivity of the prelabeled pool in 22 h), reflecting synthesis of nucleic acids (32.8% of the loss in radioactivity) and degradation to xanthine, guanine, hypoxanthine, guanosine, and inosine (49.3, 4.3, 4.1, 1.1, and 0.5% of the loss, respectively). Of the radioactivity in GuRNs, 7.9% was shifted to adenine nucleotides. The accumulation of label in xanthine indicates (in the absence of xanthine oxidase) that the main degradative pathway from GMP is that to xanthine through guanosine and guanine. The use of 8-AGuo confirmed this pathway but indicated the operation of an additional, relatively slower degradative pathway, that from GMP through IMP to inosine and hypoxanthine. Hypoxanthine was incorporated mainly into adenine nucleotide (91.5%), but a significant proportion (6%) was found in GuRNs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purine phosphoribosyltransferases of Leishmania mexicana mexicana and other flagellate protozoa

Amastigotes and cultured promastigotes of Leishmania mexicana mexicana and L. m. amazonensis, cultured promastigotes of L. donovani and L. tarentolae, and the culture forms of Crithidia fasciculata, Herpetomonas muscarum muscarum and H. m. ingenoplastis all possessed four phosphoribosyltransferase (PRTase) activities: adenine PRTase, hypoxanthine PRTase, guanine PRTase and xanthine PRTase. The enzymes of L. m. mexicana required divalent cations for activity; Mn2+ or Co2+ produced maximal activity in most cases. Hypoxanthine PRTase, guanine PRTase and xanthine PRTase from all organisms were sedimentable in part, suggesting that they may occur within glycosomes. The enzymes of L. m. mexicana cultured promastigotes were inhibited by a range of purine analogues.     

Genetic and physiological characterization of the purine salvage pathway in the archaebacterium Methanobacterium thermoautotrophicum Marburg.

The enzymes involved in the purine interconversion pathway of wild-type and purine analog-resistant strains of Methanobacterium thermoautotrophicum Marburg were assayed by radiometric and spectrophotometric methods. Wild-type cells incorporated labeled adenine, guanine, and hypoxanthine, whereas mutant strains varied in their ability to incorporate these bases. Adenine, guanine, hypoxanthine, and xanthine were activated by phosphoribosyltransferase activities present in wild-type cell extracts. Some mutant strains simultaneously lost the ability to convert both guanine and hypoxanthine to the respective nucleotide, suggesting that the same enzyme activates both bases. Adenosine, guanosine, and inosine phosphorylase activities were detected for the conversion of base to nucleoside. Adenine deaminase activity was detected at low levels. Guanine deaminase activity was not detected. Nucleoside kinase activities for the conversion of adenosine, guanosine, and inosine to the respective nucleotides were detected by a new assay. The nucleotide-interconverting enzymes AMP deaminase, succinyl-AMP synthetase, succinyl-AMP lyase, IMP dehydrogenase, and GMP synthetase were present in extracts; GMP reductase was not detected. The results indicate that this autotrophic methanogen has a complex system for the utilization of exogenous purines.     

Purine nucleoside cycles in chicken tissues

The activity of enzymes catalyzing the reactions of successive degradation to the IMP and GMP bases as well as reactions of the reutilization and degradation of the hypoxanthine and guanine bases in the chicken liver and spleen is determined. The passage rate of [8-14C]hypoxanthine label through IMP and [8-3H]guanine label through GMP is studied together with the metabolism intensity of adenine-hypoxanthine-, xanthine- and guanine-containing components and labelled acid of the acid-soluble fraction of the test tissues in experiments in vivo. The results obtained evidence for functioning of conjugated ways of hypoxanthine- and guanine-derivatives in the so-called nucleoside cycles in the chicken tissues, the activity of the guanosine cycle (GMP----guanosine----guanine----CMP) in the liver being higher than that in inosine one (IMP----inosine----hypoxanthine----IMP), whereas in the spleen, vice versa, the activity of the metabolism of hypoxanthine derivatives is higher than that of guanine derivatives.     

Guanine salvage by organ cultures of mouse tooth germs

The effect of mycophenolic acid (MPA) which inhibits the biosynthesis of guanosine monophosphate (GMP) in organ cultures of mouse tooth germs can be partially counteracted by adding guanine to the MPA cultures. This may be due to salvaging guanine by the enzyme hypoxanthine guanine phosphoribosyl transferase (HGPRT), or to competition for a common membrane carrier involved in mediated transport of both guanine and hypoxanthine in normal biosynthesis and also of MPA. Experiments were carried out to compare the effect of either hypoxanthine or guanine on the MPA-caused inhibition. While addition of guanine to the MPA cultures (MPAG) supports growth equal to controls and development of dental-enamel junction (DEJ) to a level intermediate between control and MPA the addition of hypoxanthine (MPAHX) supports growth and DEJ development not better than MPA. This indicates that guanine is salvaged by HGPRT to GMP while hypoxanthine, salvaged to inosinic acid (inosinic monophosphate, IMP) is ineffective because the MPA inhibition is on the pathway from IMP to GMP.     

Regulation of purine utilization in bacteria. VI. Characterization of hypoxanthine and guanine uptake into isolated membrane vesicles from Salmonella typhimurium.

Uptake of hypoxanthine and guanine into isolated membrane vesicles of Salmonella typhimurium TR119 was stimulated by 5'-phosphoribosyl-1'-pyrophosphate (PRPP). For strain proAB47, a mutant that lacks guanine phosphoribosyltransferase, PRPP stimulated uptake of hypoxanthine into membrane vesicles. No PRPP-stimulated uptake of guanine was observed. For strain TR119, guanosine 5'-monophosphate and inosine 5'-monophosphate accumulated intravesicularly when guanine and hypoxanthine, respectively, were used with PRPP as transport substrates. For strain proAB47, IMP accumulated intravesicularly with hypoxanthine and PRPP as transport substrates. For strain TR119, hypoxanthine also accumulated when PRPP was absent. This free hypoxanthine uptake was completely inhibited by N-ethylmaleimide, but the PRPP-stimulated uptake of hypoxanthine was inhibited only 20% by N-ethylmaleimide. Hypoxanthine and guanine phosphoribosyltransferase activity paralleled uptake activity in both strains. But, when proAB47 vesicles were sonically treated to release the enzymes, a three- to sixfold activation of phosphoribosyltransferase molecules occurred. Since proAB47 vessicles lack the guanine phsophoribosyltransferase gene product and since hypoxanthine effectively competes out the phosphoribosylation of guanine by proAB47 vesicles, it was postulated that the hypoxanthine phosphoribosyltransferase gains specificity for both guanine and hypoxanthine when released from the membrane. A group translocation as the major mechanism for the uptake of guanine and hypoxanthine was proposed.     

Crystal structures of free,IMP-, and GMP-bound Escherichia coli hypoxanthine phosphoribosyltransferase

Crystal structures have been determined for free Escherichia coli hypoxanthine phosphoribosyltransferase (HPRT) (2.9 A resolution) and for the enzyme in complex with the reaction products, inosine 5'-monophosphate (IMP) and guanosine 5'-monophosphate (GMP) (2.8 A resolution). Of the known 6-oxopurine phosphoribosyltransferase (PRTase) structures, E. coli HPRT is most similar in structure to that of Tritrichomonas foetus HGXPRT, with a rmsd for 150 Calpha atoms of 1.0 A. Comparison of the free and product bound structures shows that the side chain of Phe156 and the polypeptide backbone in this vicinity move to bind IMP or GMP. A nonproline cis peptide bond, also found in some other 6-oxopurine PRTases, is observed between Leu46 and Arg47 in both the free and complexed structures. For catalysis to occur, the 6-oxopurine PRTases have a requirement for divalent metal ion, usually Mg(2+) in vivo. In the free structure, a Mg(2+) is coordinated to the side chains of Glu103 and Asp104. This interaction may be important for stabilization of the enzyme before catalysis. E. coli HPRT is unique among the known 6-oxopurine PRTases in that it exhibits a marked preference for hypoxanthine as substrate over both xanthine and guanine. The structures suggest that its substrate specificity is due to the modes of binding of the bases. In E. coli HPRT, the carbonyl oxygen of Asp163 would likely form a hydrogen bond with the 2-exocyclic nitrogen of guanine (in the HPRT-guanine-PRib-PP-Mg(2+) complex). However, hypoxanthine does not have a 2-exocyclic atom and the HPRT-IMP structure suggests that hypoxanthine is likely to occupy a different position in the purine-binding pocket.     

Guanine uptake and metabolism in Neurospora crassa

Guanine is transported into germinated conidia of Neurospora crassa by the general purine base transport system. Guanine uptake is inhibited by adenine and hypoxanthine but not xanthine. Guanine phosphoribosyltransferase (GPRTase) activity was demonstrated in cell extracts of wild-type germinated conidia. The Km for guanine ranged from 29 to 69 micro M in GPRTase assays; the Ki for hypoxanthine was between 50 and 75 micro M. The kinetics of guanine transport differ considerably from the kinetics of GPRTase, strongly suggesting that the rate-limiting step in guanine accumulation in conidia is not that catalyzed by GPRTase. Efflux of guanine or its metabolites appears to have little importance in the regulation of pools of guanine or guanine nucleotides since very small amounts of 14C label were excreted from wild-type conidia preloaded with [8-14C]guanine. In contrast, excretion of purine bases, hypoxanthine, xanthine, and uric acid appears to be a mechanism for regulation of adenine nucleotide pools (Sabina et al., Mol. Gen. Genet. 173:31-38, 1979). No label from exogenous [8-14C]guanine was ever found in any adenine nucleotides, nucleosides, or the base, adenine, upon high-performance liquid chromatography analysis of acid extracts from germinated conidia of wild-type of xdh-l strains. The 14C label from exogenous [8-14C]guanine was found in GMP, GDP, GTP, and the GDP sugars as well as in XMP. Xanthine and uric acid were also labeled in wild-type extracts. Similar results were obtained with xdh-l extracts except that uric acid was not present. The labeled xanthine and XMP strongly suggest the presence of guanase and xanthine phosphoribosyltransferase in germinated conidia.     

Metabolism of 6-Mercaptopurine by Resistant Escherichia coli Cells

Coggin, Joseph H. (University of Chicago, Chicago, Ill.), Muriel Loosemore, and William R. Martin. Metabolism of 6-mercaptopurine by resistant Escherichia coli cells. J. Bacteriol. 92:446-454. 1966.-6-Mercaptopurine (MP) utilization as a source of purine in MP-sensitive and -resistant cultures of Escherichia coli was investigated. The label of MP-8-C(14) appeared in adenine and guanine of ribonucleic acid and deoxyribonucleic acid in sensitive and resistant cultures. Studies using MP-S(35) further demonstrated that the MP moiety was degraded, as shown by a rapid decrease in radioactivity from cells upon exposure to MP for 20 min. Enzymatic analysis showed that MP was converted to 6-mercaptopurine ribonucleotide (MPRP) by extracts derived from both sensitive and resistant cells. Resistant cell preparations, however, degraded MPRP to inosine monophosphate (IMP) rapidly when compared with analogue degradation by sensitive cells. Inosineguanosine-5'-phosphate pyrophosphorylase from resistant cells did not catalyze the synthesis of IMP from hypoxanthine when the cells were cultured in the presence of MP, but these enzyme preparations actively converted guanine to guanosine monophosphate (GMP). Pyrophosphorylase derived from resistant cells cultured in medium without MP catalyzed the conversion of hypoxanthine to IMP and also guanine to GMP. These observations suggest that inosine-guanosine-5'-phosphate pyrophosphorylase is composed of two distinct enzymes. The mode of resistance to MP in E. coli is related to an enhancement of the enzymatic degradation of MPRP to the pivotal purine intermediate, IMP.     

High Km soluble 5'-nucleotidase from human placenta. Properties and allosteric regulation by IMP and ATP

A human placental soluble "high Km" 5'-nucleotidase has been separated from "low Km" 5'-nucleotidase and nonspecific phosphatase by AMP-Sepharose affinity chromatography. The enzyme was purified 8000-fold to a specific activity of 25.6 mumol/min/mg. The subunit molecular mass is 53 kDa, and the native molecular mass is 210 kDa, suggesting a tetrameric structure. Soluble high Km 5'-nucleotidase is most active with IMP and GMP and their deoxy derivatives. IMP is hydrolyzed 15 times faster than AMP. The enzyme has a virtually absolute requirement for magnesium ions and is regulated by them. Purine nucleoside 5'-triphosphates strongly activate the enzyme with the potency order dATP greater than ATP greater than GTP. 2,3-Diphosphoglycerate activates the enzyme as potently as ATP. Three millimolar ATP decreased the Km for IMP from 0.33 to 0.09 mM and increased the Vmax 12-fold. ATP activation was modified by the IMP concentration. At 20 microM IMP the ATP-dependent activation curve was sigmoidal, while at 2 mM IMP it was hyperbolic. The A0.5 values for ATP were 2.26 and 0.70 mM, and the relative maximal velocities were 32.9 and 126.0 nmol/min, respectively. Inorganic phosphate shifts the hyperbolic substrate velocity relationship for IMP to a sigmoidal one. With physiological concentrations of cofactors (3 mM ATP, 1-4 mM Pi, 150 mM KCl) at pH 7.4, the enzyme is 25-35 times more active toward 100 microM IMP than 100 microM AMP. These data show that: (a) soluble human placental high Km 5'-nucleotidase coexists in human placenta with the low Km enzyme; (b) under physiological conditions the enzyme favors the hydrolysis of IMP and is critically regulated by IMP, ATP, and Pi levels; and (c) kinetic properties of ATP and IMP are each modified by the other compound suggesting complex interaction of the associated binding sites.     

Effects of benzimidazole on the purine and pyrimidine metabolism of yeast.

Yeast cells inhibited by benzimidazole accumulate hypoxanthine with associated efflux of xanthine. Unlike control cells, inhibited cells contain no detectable free UMP and CMP. Benzimidazole decreases uptake of [8-14C]hypoxanthine into the intracellular pool of hypoxanthine and xanthine but causes radioactive xanthine to accumulate in the medium. In inhibited cultures there is a threefold increase in incorporation of [8-14C]hypoxanthine into the total (intracellular plus extracellular) xanthine. Uptake of [8-14C]hypoxanthine into free nucleotides and into bound adenine and guanine was inhibited by 70%. Uptake of [U-14C]glycine into IMP, AMP, GMP, DNA and RNA was also substantially decreased. Incorporation of [2-14C]uracil into the intracellular uracil pool was inhibited by 30% and into free uridine and cytidine by over 90%. Benzimidazole inhibited incorporation of [8-3H]IMP into AMP and GMP, and decreased substantially the activity of glutamine-amidophosphoribosyltransferase (EC 2.4.2.14). Yeast cultures were shown to N-ribotylate benzimidazole. Results are consistent with benzimidazole inhibiting yeast growth by competing for P-rib-PP and so depriving other ribotylation processes such as the 'salvage' pathways and de novo synthesis of purines and pyrimidines.     

Hypoxanthine-guanine exchange by intact human erythrocytes

The uptake and release of [14C]hypoxanthine by human erythrocytes, suspended in a tris(hydroxymethyl)aminomethane (Tris)-glucose-NaCl isotonic medium (pH 7.4), have been studied at 37 degrees C. The uptake of hypoxanthine, mediated by its incorporation into inosine 5'-monophosphate (IMP), was markedly stimulated by preincubating the cells in phosphate-buffered saline. After a lag time, [14C]IMP-enriched erythrocytes released [14C]hypoxanthine in the medium. Formycin B, at concentrations known to inhibit purine nucleoside phosphorylase in intact erythrocytes, affected hypoxanthine uptake and release and led to an increase in the intracellular concentration of inosine, suggesting that the main catabolic path of IMP is the sequential degradation of the nucleotide to inosine and hypoxanthine. The addition of guanine to a suspension of [14C]IMP-enriched erythrocytes led to an increase in the rate of [14C]hypoxanthine release, which was unaffected by the presence of formycin B. During the guanine-induced hypoxanthine release, guanine was taken up by the cells as GMP. These results suggest that the presence of guanine in the incubation medium activates a catabolic path in human erythrocytes leading to IMP degradation without formation of inosine.     

Specificities and pH profiles of adenine and hypoxanthine–guanine–xanthine phosphoribosyltransferases (nucleotide synthases) of the thermoacidophile archaeon Sulfolobus solfataricus

Two open reading frames in the genome of Sulfolobus solfataricus (SSO2341 and SSO2424) were cloned and expressed in E. coli. The protein products were purified and their enzymatic activity characterized. Although SSO2341 was annotated as a gene (gpT-1) encoding a 6-oxopurine phosphoribosyltransferase (PRTase), the protein product turned out to be a PRTase highly specific for adenine and we suggest that the reading frame should be renamed apT. The other reading frame SSO2424 (gpT-2) proved to be a true 6-oxopurine PRTase active with hypoxanthine, xanthine and guanine as substrates, and we suggest that the gene should be renamed gpT. Both enzymes exhibited unusual profiles of activity versus pH. The adenine PRTase showed the highest activity at pH 7.5–8.5, but had a distinct peak of activity also at pH 4.5. The 6-oxo PRTase showed maximal activity with hypoxanthine and guanine around pH 4.5, while maximal activity with xanthine was observed at pH 7.5. We discuss likely reasons why SSO2341 in S. solfataricus and similar open reading frames in other Crenarchaeota could not be identified as genes encoding APRTase.     

Inhibition of purine phosphoribosyltransferases from Ehrlich ascites-tumour cells by purine nucleotides

1. The purine bases adenine, hypoxanthine and guanine were rapidly incorporated into the nucleotide fraction of Ehrlich ascites-tumour cells in vivo. 2. The reaction of 5'-phosphoribosyl pyrophosphate with adenine phosphoribosyltransferase from ascites-tumour cells (K(m) 6.5-11.9mum) was competitively inhibited by AMP, ADP, ATP and GMP (K(i) 7.5, 21.9, 395 and 118mum respectively). Similarly the reactions of 5'-phosphoribosyl pyrophosphate with both hypoxanthine phosphoribosyltransferase and guanine phosphoribosyltransferase (K(m) 18.4-31 and 37.6-44.2mum respectively) were competitively inhibited by IMP (K(i) 52 and 63.5mum) and by GMP (K(i) 36.5 and 5.9mum). 3. The nucleotides tested as inhibitors did not appreciably compete with the purine bases in the phosphoribosyltransferase reactions. 4. It was postulated that the purine phosphoribosyltransferases of Ehrlich ascites-tumour cells may be effectively separated from the adenine nucleotide pool of these cells.     

Distinct roles for recombinant cytosolic 5'-nucleotidase-I and -II in AMP and IMP catabolism in COS-7 and H9c2 rat myoblast cell lines

Catabolism of AMP during ATP breakdown produces adenosine, which restores energy balance. Catabolism of IMP may be a key step regulating purine nucleotide pools. Two, cloned cytosolic 5'-nucleotidases (cN-I and cN-II) have been implicated in AMP and IMP breakdown. To evaluate their roles directly, we expressed recombinant pigeon cN-I or human cN-II at similar activities in COS-7 or H9c2 cells. During rapid (more than 90% in 10 min) or slower (30-40% in 10 min) ATP catabolism, cN-I-transfected COS-7 and H9c2 cells produced significantly more adenosine than cN-II-transfected cells, which were similar to control-transfected cells. Inosine and hypoxanthine concentrations increased only during slower ATP catabolism. In COS-7 cells, 5'-nucleotidase activity was not rate-limiting for inosine and hypoxanthine production, which was therefore unaffected by cN-II- and actually reduced by cN-I- overexpression. In H9c2 cells, in which 5'-nucleotidase activity was rate-limiting, only cN-II overexpression accelerated inosine and hypoxanthine formation. Guanosine formation from GMP was also increased by cN-II. Our results imply distinct roles for cN-I and cN-II. Under the conditions tested in these cells, only cN-I plays a significant role in AMP breakdown to adenosine, whereas only cN-II breaks down IMP to inosine and GMP to guanosine.     

Characteristics of purine nucleotide metabolism and GMP-reductase activity in chicken tissues

The [3H]guanosine and [3H]guanine label is shown to be distributed unevenly in the purine components of chicken tissues. 60 min after isotope administration about 80% of radioactivity is localized in xanthine and uric acid in the liver and duodenum, that agrees with high activity of purine nucleoside phosphorylase (EC 2.4.2.1) and guanine deaminase (EC 3.5.4.3). At the same time over 50% of label is found in the spleen in adenine nucleotides of the pool, RNA as well as in hypoxanthine and only 20% in oxypurines. Such a distribution of the label is in direct correlation with the activity of GMP-reductase (EC 1.6.6.8) catalyzing the reduction deamination of GMP in IMP.     

Human brain hypoxanthine guanine phosphoribosyltransferase: structural and functional comparison with erythrocyte hypoxanthine guanine phosphoribosyltransferase

A rapid and simple method, based on GMP Sepharose affinity chromatography, was used for the purification of human brain hypoxanthine guanine phosphoribosyltransferase. A single protein band was detected by polyacrylamide gel electrophoresis of the native purified enzyme. A subunit molecular weight of 25,000 was estimated by SDS gel electrophoresis. The Km values for hypoxanthine and phosphoribosyl pyrophosphate were 50 and 111 microM, respectively. The Ki values for GMP and IMP with phosphoribosyl pyrophosphate were 21 and 37 microM, respectively. The purified enzyme from human brain did not differ significantly from the human erythrocyte one in amino acid composition. The brain and erythrocyte hypoxanthine guanine phosphoribosyltransferases showed complete immunochemical identity on Ouchterlony double diffusion.