Gastrointestinal absorption of folates and antifolates.
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Biomedical subjects
Publications and source records attributed to G J Dhar.
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Porphyrin and iron distributions in the liver biopsies of patients with Porphyria cutanea tarda (PCT) were investigated by comparison of composite photographs from fluorescence microscope and after iron stain. Three distinct areas are visible: 1. red fluorescent areas with porphyrins; 2. blue areas corresponding to iron; 3. areas with neither iron nor porphyrin. The areas with iron or porphyrin do not overlap, therefore, this experiment indicates lack of direct correlation between iron and prophyrin distribution in livers of PCT patients.
Transitory renal failure occurred in a patient with acute intermittent porphyria in clinical remission following i.v. administration of 1 000 mg hematin. The clinical and biochemical picture suggested "acute tubular necrosis", which was followed by a prompt and complete return of renal function without any late sequelae. The renal failure is thought to have resulted from the presence of circulating free hematin, formed as a result of rapid administration of such a relatively large amount. Such a complication has not occurred in patients given hematin for acute porphyric relapse, in whom much smaller amounts have been infused.
Sulfasalazine (salicylazosulfapyridine), an agent widely used for the treatment of ileitis and colitis, is also a competitive inhibitor of intestinal folate transport (1, 2). The mechanism of action of sulfasalazine remains uncertain. To further explore the mechanism of sulfasalazine action, the interaction of the drug with the folate recognition site was tested with three enzymes: dihydrofolate reductase, methylenetetrahydrofolate reductase, and serine transhydroxymethylase, each catalyzing a reaction involving a different folate derivative. Each of these enzymes was inhibited by sulfasalazine in the same concentration range as that previously observed to inhibit intestinal folate transport; the kinetic data are consistent with a competitive mode of inhibition. Specificity of inhibition was demonstrated by the finding that the reduction of the pteridine ring of pteroylheptaglutamic acid by dihydrofolate reductase was subject to inhibition, whereas the hydrolysis of the gamma-glutamyl peptide side chain by chicken pancreas conjugase was not affected. These results are interpreted to indicate that sulfasalazine interferes with a folate recognition site which is common to these enzymes and to the intestinal transport system. Sulfasalazine, therefore, has certain properties of an antifolate drug.
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The present study was carried out in five cases of hepatic porphyria, including three of acute intermittent porphyria, one of variegate porphyria, and one of porphyria cutanea tarda in clinical remission. In two cases of acute intermittent porphyria (in relapse), a marked lowering effect on serum and urine porphobilinogen and delta-aminolevulinic acic was observed, together with prompt and gratifying clinical improvement. In a third case, in chemical remission but with longstanding psychoneurosis, no significant effects were noted, nor were any observed in the case of porphyria cutanea tarda. Although clinical improvements occurred in the case of variegate porphyria, the results were inconclusive for reasons given. Hematin was generally well tolerated. Preliminary reference is made to a transitory renal injury, without sequelae, where an excess of hematin was given in relation to time. Limits of tolerance are proposed. In the light of these observations the basic mechanism of the acute attack is diccussed.
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