Platelet function is influenced by the platelet thioldisulfide balance. Platelet activation resulted in 440% increase in surface protein thiol groups. Two proteins that presented free thiol(s) on the activated platelet surface were protein-disulfide isomerase (PDI) and glycoprotein 1b␣ (GP1b␣). PDI contains two active site dithiols/disulfides. The active sites of 26% of the PDI on resting platelets was in the dithiol form, compared with 81% in the dithiol form on activated platelets. Similarly, GP1b␣ presented one or more free thiols on the activated platelet surface but not on resting platelets. Anti-PDI antibodies increased the dissociation constant for binding of vWF to platelets by ϳ50% and PDI and GP1b␣ were sufficiently close on the platelet surface to allow fluorescence resonance energy transfer between chromophores attached to PDI and GP1b␣. Incubation of resting platelets with anti-PDI antibodies followed by activation with thrombin enhanced labeling and binding of monoclonal antibodies to the N-terminal region of GP1b␣ on the activated platelet surface. These observations indicated that platelet activation triggered reduction of the active site disulfides of PDI and a conformational change in GP1b␣ that resulted in exposure of a free thiol(s).The platelet thiol-disulfide balance is important for platelet function. Perturbation of platelet thiol status effects platelet aggregation and release. The low M r thiol compounds, reduced glutathione (GSH), cysteine, and 6-mercaptopurine, inhibit platelet aggregation induced by several agonists, while the disulfide-bond reducing agents dithiothreitol and ␤-mercaptoethanol promote aggregation (1). In addition, reaction of platelet sulfhydryl groups with the thiol specific compounds, diamide and N-ethylmaleimide, inhibits in vitro aggregation and clot retraction (2-5). These results imply that certain platelet thiol groups are critical for platelet aggregation. Furthermore, the observation that specific depletion of platelet GSH by 1-chloro-2,4-dinitrobenzene only marginally effects platelet aggregability implies that the critical thiol groups are associated with protein (6). In support of this notion, Yamada et al. (7) have shown that the anti-platelet aggregation actions of 2,2Ј-dithiobis(N-2-hydroxypropylbenzamide) are mediated through interaction of the compound with platelet protein thiol groups.Protein-disulfide isomerase (PDI) 1 is a noncovalent homodimer with a subunit molecular mass of 57 kDa that catalyzes thiol-disulfide interchanges that can result in formation, reduction, or rearrangement of protein disulfide bonds. It is generally considered that PDI is important for proper folding and disulfide bonding of nascent proteins in the endoplasmic reticulum (8 -10). PDI also functions as the ␤ subunits of prolyl-4-hyroxylase (11, 12) and the ␤ subunit of triglyceride transfer protein complex (13,14). Bovine aortic endothelial cells (15), rat hepatocytes (16), rat pancreatic cells (17), and human B cells (18,19) secrete PDI which associates with the cell su...

The treatment of homocystinuria that is not responsive to pyridoxine is not usually biochemically or clinically successful, and vascular, ocular, and skeletal complications commonly supervene. Persistent marked homocysteinemia appears to be the most important biochemical disturbance leading to these complications. Ten patients with cystathionine beta-synthase deficiency that was not responsive to pyridoxine and one patient with homocystinuria due to a defect in cobalamin metabolism were treated with 6 g daily of betaine added to conventional therapy, to improve homocysteine remethylation. All patients had a substantial decrease in plasma total homocysteine levels (P less than 0.001) and an increase in total cysteine levels (P less than 0.001). Changes in plasma methionine concentrations were variable. Fasting levels of plasma amino acids became normal in two patients, and in six there was immediate clinical improvement. There were no unwanted effects. We conclude that treatment of homocystinuria that is not responsive to pyridoxine and of disorders of homocysteine remethylation should include betaine in adequate doses to ensure maximum lowering of elevated plasma homocysteine levels.

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