The incorportion of 15Ninto adenine nucleotides and their formation from inosine monophosphate by skeletal-muscle preparations

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

Energy-rich phosphate compounds in bull semen: comparison of their metabolism with anaerobic heat production and impedance change frequency

1. The concentrations of nucleotides and other energy rich phosphate compounds in bull spermatozoa and bull semen were examined at different times during anaerobic incubation at 37 °C. The variations in the concentrations of these compounds were compared with sperm heat production and impedance change frequency during anaerobic incubation. 2. Immediately after collection there were 129 ± 14 p μ moles ATP , 120 ± 15.5 p μ moles ADP and 271 ± 22 p μ moles AMP + HR (in approximately equal quantities) per sperma­tozoon. (10 3 p μ moles x sperm density = μ moles/ml. semen.) The correlation between ATP and ADP concentration per spermatozoon and the percentage of live spermatozoa in the sample was 0.89, P < 0.05. 3. During the whole period of incubation, which lasted 120 to 430 min, the following changes in sperm nucleotide content took place; ( a ) ATP decreased; ( b ) ADP decreased; ( c ) labile-P decreased; ( d ) AMP + HR increased; ( e ) there was no inverse relationship between ATP and ADP . 4. During the first hour of incubation, when there was a sharp fall in heat production and impedance change frequency, there was no comparable decline in ATP , ADP , or labile-P. In one experiment there was a 55% increase in ATP during this period. 5. Contrary to recent reports, compounds which were not nucleotides but which contained highly labile phosphate groups were found in bull semen. Their concentration was about 250 p μ moles/spermatozoon, of which 50 p μ moles was creatine phosphate. The possibility that the rest was acetyl phosphate or 1, 3-diphosphoglycerate is discussed. 6. When the anaerobic heat production of bull semen fell to zero, which in these experiments we established was not due to exhaustion of fructose in the external medium but to the low pH, ≼5.5, of the seminal plasma, there were still significant amounts of nucleotides present in the spermatozoa.

The purine nucleotide cycle and ammonia formation from glutamine by rat kidney slices

To test the significance of the purine nucleotide cycle in renal ammoniagenesis, studies were conducted with rat kidney cortical slices using glutamate or glutamine labelled in the alpha-amino group with 15N. Glucose production by normal kidney slices with 2 mM-glutamine was equal to that with 3 mM-glutamate. With L-[15N]glutamate as sole substrate, one-third of the total ammonia produced by kidney slices was labelled, indicating significant deamination of glutamate or other amino acids from the cellular pool. Ammonia produced from the amino group of L-[alpha-15N]glutamine was 4-fold higher than from glutamate at similar glucose production rates. Glucose and ammonia formation from glutamine by kidney slices obtained from rats with chronic metabolic acidosis was found to be 70% higher than by normal kidney slices. The contribution of the amino group of glutamine to total ammonia production was similar in both types of kidneys. No 15N was found in the amino group of adenine nucleotides after incubation of kidney slices from normal or chronically acidotic rats with labelled glutamine. Addition of Pi, a strong inhibitor of AMP deaminase, had no effect on ammonia formation from glutamine. Likewise, fructose, which may induce a decrease in endogenous Pi, had no effect on ammonia formation. The data obtained suggest that the contribution of the purine nucleotide cycle to ammonia formation from glutamine in rat renal tissue is insignificant.

Mammalian adenylosuccinate lyase. Participation in the conversion of 2'-dIMP and beta-D-arabinosyl-IMP to adenine nucleotides

Adenylosuccinate synthetase (EC 6.3.4.4) from rabbit muscle efficiently catalyzes the formation of 2'-deoxyadenylosuccinate and beta-D-arabinosylade-nylosuccinate from 2'-dIMP and beta-D-arabinosylIMP (Spector, T. and Miller, RL. (1976) Biochim. Biophys. Acta 445, 509-517). These novel analogs of adenylosuccinate were synthesized with this enzyme and their kinetic constants were determined with adenylosuccinate lyase purified from Ehrlich ascites cells. 2'-Deoxyadenylosuccinate and beta-D-arabinosyladenylosuccinate were readily cleaved to 2'-dAMP and beta-D-arabinosylAMP, respectively. Their Km values were similar to that of adenylosuccinate (3-6 micronM) and their substrate efficiencies (V/Km) were 120 for 2-deoxyadenylosuccinate and 32 for beta-D-arabinosyl-adenylosuccinate, compared to a value of 100 for adenylosuccinate. The products of the reactions, 2'-dAMP and beta-D-arabinosylAMP, were competitive inhibitors with Ki values of 5 and 87 micronM, respectively. ATP and ADP were considerably weaker competitive inhibitors with Ki values of 200-300 micronM. IMP, GMP, xanthosine 5'-monophosphate, 6-thioIMP and 6-thioGMP had Ki values greater than 200 micronM.

Ammonia production in muscle: the purine nucleotide cycle

Experiments are reported which throw new light on the problem of ammonia production by muscle and, probably, by other tissues.

The mass-spectrometirc identification of hypoxanthine and xanthine ('oxypurines') in skeletal musce from two patients with congenital xanthine oxidase deficiency (xanthinuria)

1. The presence of hypoxanthine and xanthine in the skeletal muscle of two patients with congenital xanthine oxidase deficiency (xanthinuria) was demonstrated by high-resolution mass spectrometry. 2. Evidence was obtained for the presence of a trace of hypoxanthine only in normal muscle. 3. Dry pulverized tissue was introduced directly into the mass spectrometer and preliminary chemical processing of the tissue was therefore unnecessary. 4. The criteria for the mass-spectrometric identification of hypoxanthine and xanthine in the tissue and the significance of the observations are discussed.