Joel Moss

Effect of salicylates on histamine and L-histidine metabolism. Inhibition of imidazoleacetate phosphoribosyl transferase.

In man and other animals, urinary excretion of the histidine and histamine metabolite, imidazoleacetate, is increased and that of its conjugated metabolite, ribosylimidazoleacetate, decreased by salicylates. Imidazoleacetate has been reported to produce analgesia and narcosis. Its accumulation as a result of transferase inhibition could play a part in the therapeutic effects of salicylates. To determine the locus of salicylate action, we have investigated the effect of anti-inflammatory drugs on imidazoleacetate phosphoribosyl transferase, the enzyme that catalyzes the ATP-dependent conjugation of imidazoleacetate with phosphoribosylpyrophosphate. As little as 0.2 mM aspirin produced 50% inhibition of the rat liver transferase. In vivo, a 30% decrease in the urinary excretion of ribosylimidazoleacetate has been observed with plasma salicylate concentrations of 0.4 mM. The enzyme was also inhibited by sodium salicylate but not by salicylamide, sodium gentisate, aminopyrine, phenacetin, phenylbutazone, or indomethacin. The last four drugs have been shown previously not to alter the excretion of ribosylimidazoleacetate when administered in vivo. Since both the drug specificity and inhibitory concentrations are similar in vivo and in vitro, it seems probable that the effect of salicylates on imidazoleacetate conjugation results from inhibition of imidazoleacetate phosphoribosyl transferase.

Characterization of the Family of Mammalian Genes Encoding ADP- ribosylation Factors

ADP-ribosylation factors (ARFs) are ~20-kDa guanine nucleotide-binding proteins that stimulate the in vitro cholera toxin-catalyzed ADP-ribosylation of the α subunit of Gs (the stimulatory GTP-binding protein of the adenylyl cyclase system) (Kahn and Gilman, 1984, 1986; Bobak et al., 1990). Membrane (mARF) and soluble (sARF I and sARF II) ADP-ribosylation factors that enhance cholera toxin-catalyzed ADP-ribosylation of Gsα, and simple guanidino compounds were identified in bovine brain (Tsai et al., 1987, 1988). Subsequent molecular cloning of the cDNAs that encode the ARFs revealed the existence of a larger family of related guanine nucleotide-binding proteins. To date, six mammalian ARF cDNAs (ARFs 1 to 6) have been isolated from bovine and/or human libraries (Sewell and Kahn, 1988; Price et al., 1988; Bobak et al., 1989; Monaco et al., 1990; Tsuchiya et al., 1991). Based on the deduced amino acid sequences, mammalian ARFs represent a family of 20kDa guanine nucleotide-binding proteins clearly different from members of the ras and ras-like (20-25 kDa) superfamily. In particular, the signature sequences of the regions believed to be involved in guanine nucleotide binding and GTP hydrolysis in the ARF proteins more closely resemble those in the heterotrimeric G protein a subunits than those found in the ras, rho, rac,rap, ral, and rab families (Price et al., 1990). The GTP-hydrolysis domain GXXXXGK is completely conserved across the mammalian ARFs as GLDAAGK. The sequence DVGG which forms the DXXG consensus sequence that has been proposed to coordinate binding to Mg2+ and the β phosphate of GDP and the sequence NKQD (the NKXD consensus sequence) which is thought to contribute to the specificity of interaction with the purine ring of GTP, are found in all the deduced ARF sequences (Price et al., 1990).