P. D. Dawkins

Salicylate and enzymes

Salicylate inhibits the activities of a number of cellular enzymes and in some instances the mechanisms of inhibition have been established (Smith, 1968a). Reported inhibitory actions of the salicylate ion on important enzyme systems in vitro are now reviewed and assessed in relation to the known clinical and toxic effects of the drug.

The effect of sodium salicylate on the binding of l‐tryptophan to serum proteins

In human serum, l‐tryptophan is the only amino‐acid bound to protein. Salicylate causes a release of tryptophan from its binding sites on human serum proteins and bovine albumin. Some implications of this finding are discussed.

The displacement of l‐tryptophan and dipeptides from bovine albumin in vitro and from human plasma in vivo by antirheumatic drugs

l‐Tryptophan occurs in a protein‐bound and an unbound form in serum from normal subjects. The amino‐acid is displaced from its binding sites in vitro by salicylate, phenylbutazone, indomethacin, prednisolone, chloroquine and gold salts and is virtually absent in serum obtained from patients with rheumatoid arthritis receiving therapy with antirheumatic drugs. Some dipeptides bind to bovine albumin in vitro and are displaced by salicylate. All the drugs displace l‐phenylalanyl‐l‐phenylalanine from its binding to normal human serum in vitro.

The binding of indomethacin, salicylate and phenobarbitone to human whole blood in vitro.

The binding of indomethacin, salicylate and phenobarbitone to human whole blood, plasma and red cells has been determined by equilibrium dialysis. The red cells bound appreciable proportions of salicylate and phenobarbitone but not indomethacin. It is concluded that prediction of the extent of the drug binding to the circulating blood should be made from the results obtained with whole blood, red cells and plasma.

The distribution of salicylate in mouse tissues after intraperitoneal injection

The concentrations of salicylate and its principal metabolites were measured in blood, liver, brain, kidney, heart, spleen, diaphragm and skeletal muscle after the intraperitoneal injection of a fixed amount of radioactive salicylate and varying doses of unlabelled salicylate. The patterns of distribution of salicylate in the various organs with time were similar, the peak level being attained in 30–60 min after injection. Salicylate was eliminated from the blood after 8 hr but persisted in liver in measurable amounts up to 24 hr.

The mechanism of the inhibition of dehydrogenases by salicylate.

Salicylate inhibits rabbit muscle lactate dehydrogenase, horse liver alcohol dehydrogenase, pig heart malate dehydrogenase and pig heart isocitrate dehydrogenase in vitro. The inhibitions are reversible, involving competition with nad, nadh2 or nadp. The results are discussed with reference to some of the in vivo actions of the drug.

The effect of sodium salicylate on the binding of long-chain fatty acids to plasma proteins

Salicylate causes a release of palmitic, stearic, oleic and linolenic acids from their binding sites on human plasma proteins and bovine albumin. The implications of this finding with respect to other effects of salicylate on fatty acid metabolism are discussed.

The relation between circulating and tissue concentrations of salicylate in the mouse in vivo

The blood and water contents of mouse liver, brain, kidney, heart, spleen and skeletal muscle were measured and used to correct observed values for the salicylate concentrations in these tissues after the intraperitoneal injection of the drug. The binding of salicylate in vitro to mouse whole blood, liver, kidney and brain was studied. It was concluded that blood, liver and kidney but not the other tissues, bind the drug in vivo.

Inhibition of protein biosynthesis in mouse liver by salicylate

Abstract The incorporation of l -[U-14C]leucine and l -[ring-2-U14C]histidine into the protein of a post-mitochondrial supernatant fraction from mouse liver is inhibited by salicylate concentrations of 1 mM and above. The intraperitoneal injection of 500–600 mg/kg of salicylate in the mouse causes the following: an increase in the liver concentration of leucine but not that of histidine; a decrease in the incorporation of radioactive leucine and histidine into liver protein in vitro; an inhibition of the incorporation of radioactivity from intraperitoneally injected leucine into liver protein in vivo; a decrease in the transfer of α-aminoisobutyrate from the peritoneum to the blood but not from the circulation to the tissues and an inhibition of protein biosynthesis in the liver after the intravenous administration of radioactive leucine and histidine. It is concluded that salicylate interferes with protein synthesis in mouse liver both in vitro and in vivo.

The relation between clinical anti‐inflammatory activity and the displacement of l‐tryptophan and a dipeptide from human serum in vitro

It has been reported (McArthur, Dawkins & Smith, 1971) that salicylate, phenylbutazone, indomethacin, prednisolone, chloroquine and gold salts share a common action in displacing L-tryptophan, and several dipeptides, particularly L-phenylalanyl-L-phenylalanine, from their binding sites to bovine albumin and to human serum proteins in vitro. We have observed that this action is shared by two other potent anti-inflammatory substances, mefenamic and flufenamic acids, but not by other drugs that resemble the commonly used antirheumatic remedies in being administered over long periods of time. We have found that phenobarbitone, penicillin V, ampicillin, cloxacillin, ascorbic acid or paracetamol, when studied in a range of concentrations at least twice those encountered in the circulation during therapy, did not displace either L-tryptophan or L-phenylalanyl-L-phenylalanine, from human serum when this was investigated by the techniques described by McArthur, Dawkins & Smith (1971). The failure of paracetamol to show this effect is of particular interest since the drug does not possess clinical anti-inflammatory activity (Fremont-Smith & Bayles, 1965 ; Boardman & Hart, 1967) but is frequently administered as an analgesic in rheumatoid arthritis. It has been proposed (McArthur, Dawkins & others, 1971) that antirheumatic drugs act by displacing certain peptides from their binding sites to circulating proteins and that the free fractions of these peptides protect susceptible tissues against the effect of chronic inflammatory reactions. In patients with rheumatoid arthritis and and similar disorders the peptides are bound to an abnormal extent to the serum proteins and the drugs act by restoring the equilibrium to normal. The behaviour of L-tryptophan and L-phenylalanyl-L-phenylalanine mimics that of the hypothetical protective peptides. The present report provides additional evidence in favour of this hypothesis since it shows that a displacing action is common to antirheumatic drugs but is not given by other drugs that bind to human serum proteins and are administered in divided doses over similar periods of time.