Total homocysteine is defined as the sum of all homocysteine species in plasma/serum, including free and protein-bound forms. In the present review, we compare and evaluate several techniques for the determination of total homocysteine. Because these assays include the conversion of all forms into a single species by reduction, the redistribution between free and protein-bound homocysteine through disulfide interchange does not affect the results, and total homocysteine can be measured in stored samples. Total homocysteine in whole blood increases at room temperature because of a continuous production and release of homocysteine from blood cells, but artificial increase is low if the blood sample is centrifuged within 1 h of collection or placed on ice. Different methods correlate well, and values between 5 and 15 mumol/L in fasting subjects are considered normal. Total homocysteine in serum/plasma is increased markedly in patients with cobalamin or folate deficiency, and decreases only when they are treated with the deficient vitamin. Total homocysteine is therefore of value for the diagnosis and follow-up of these deficiency states and may compensate for weaknesses of the traditional laboratory tests. In addition, total homocysteine is an independent risk factor for premature cardiovascular diseases. These disorders justify introduction of the total homocysteine assay in the routine clinical chemistry laboratory.

https://www.folk.uib.no/mfapu/Pages/papers%20pdf/1993/ueland_2000_cc_39_1764.pdf

Total homocysteine in plasma or serum: methods and clinical applications.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy.

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches.

A thorough study was done to test the reactivity of the Folin−Ciocalteu (F-C) reagent toward various compound classes. Over 80 compounds were tested. Compound classes included phenols, thiols, vitamins, amino acids, proteins, nucleotide bases, unsaturated fatty acids, carbohydrates, organic acids, inorganic ions, metal complexes, aldehydes, and ketones. All phenols, proteins, and thiols tested were reactive toward the reagent. Many vitamin derivatives were also reactive, as were the inorganic ions Fe+2, Mn2+, I−, and SO32−. Other compounds showing reactivity included the nucleotide base guanine and the trioses glyceraldehyde and dihydroxyacetone. Copper complexation enhanced the reactivity of salicylate derivatives toward the reagent, whereas zinc complexation did not. Several amino acids and sugars that were reported to be reactive toward the F-C reagent in earlier studies were found not to be reactive in this study, at least in the concentrations used. Reaction kinetics of each compound with the F-C rea...

Thorough study of reactivity of various compound classes toward the Folin-Ciocalteu reagent.

Role of CYP2E1 in the Hepatotoxicity of Acetaminophen

Utilisation de cyanamide marque au 15 N et au 13 C pour mettre en evidence l'intermediaire N-hydroxycyanamide donnant par decomposition le nitroxyle (HN=0), inhibiteur de l'aldehyde deshydrogenase (ALDH)

Evidence for nitroxyl in the catalase-mediated bioactivation of the alcohol deterrent agent cyanamide

A number of 2-alkyl- and 2-aryl-substituted thiazolidine-4(R)-carboxylic acids were evaluated for their protective effect against hepatotoxic deaths produced in mice by LD90 doses of acetaminophen. 2(RS)-Methyl-, 2(RS)-n-propyl-, and 2(RS)-n- pentylthiazolidine -4(R)-carboxylic acids (compounds 1b,d,e, respectively) were nearly equipotent in their protective effect based on the number of surviving animals at 48 h as well as by histological criteria. 2(RS)-Ethyl-, 2(RS)-phenyl-, and 2(RS)-(4-pyridyl)thiazolidine-4(R)-carboxylic acids (compounds 1c,f,g) were less protective. The enantiomer of 1b, viz., 2(RS)- methylthiazolidine -4(S)-carboxylic acid (2b), was totally ineffective in this regard. Thiazolidine-4(R)-carboxylic acid (1a), but not its enantiomer, 2a, was a good substrate for a solubilized preparation of rat liver mitochondrial proline oxidase [Km = 1.1 x 10(-4) M; Vmax = 5.4 mumol min-1 (mg of protein)-1]. Compound 1b was not a substrate for proline oxidase but dissociated to L-cysteine in this system. At physiological pH and temperature, the hydrogens on the methyl group of 1b underwent deuterium exchange with solvent D2O (k1 = 2.5 X 10(-5) s), suggesting that opening of the thiazolidine ring must have taken place. Indeed, 1b labeled with 14C in the 2 and methyl positions was rapidly metabolized by the rat to produce 14CO2, 80% of the dose being excreted in this form in the expired air after 24 h. It is suggested that these 2-substituted thiazolidine-4(R)-carboxylic acids are prodrugs of L-cysteine that liberate this sulfhydryl amino acid in vivo by nonenzymatic ring opening, followed by solvolysis.

2-Substituted thiazolidine-4(R)-carboxylic acids as prodrugs of L-cysteine. Protection of mice against acetaminophen hepatotoxicity

Eight prodrugs of L-cysteine (1a-h) were synthesized by the condensation of the sulfhydryl amino acid with naturally occurring aldose monosaccharides containing three, five, and six carbon atoms The resulting 2-(polyhydroxyalkyl)thiazolidine-4(R)-carboxylic acids (TCAs) are capable of releasing L-cysteine and the sugars by nonenzymatic ring opening and hydrolysis Thus, when added to rat hepatocyte preparations in vitro, these TCAs (10 mM) raised cellular glutathione (GSH) levels 12-21-fold relative to controls On the basis of this finding, the cysteine prodrugs were tested as protective agents against acetaminophen-induced hepatotoxicity in a mouse model The TCA derived from D-ribose and L-cysteine (RibCys, 1d) showed the greatest therapeutic promise of the series, with a 100% (12/12) survival profile compared to 17% without treatment However, the degree of stimulation of GSH production in rat hepatocytes by these prodrugs did not correlate with the extent of protection afforded in mice, suggesting that pharmacokinetic parameters must supervene in vivo To evaluate the effect of increased lipid solubility, we prepared prodrugs 2a-c by using peracetylated aldehydic sugars in the condensation reaction These compounds, however, displayed acute toxicity to mice, possibly due to liberation of the acetylated sugars themselves Nevertheless, the efficacy of the unacetylated TCAs, and RibCys (1d) in particular, suggests that the prodrug approach for the delivery of L-cysteine to the liver represents a viable means of augmenting existing detoxication mechanisms in protecting cells against xenobiotic substances that are bioactivated to toxic, reactive metabolites

Prodrugs of L-cysteine as protective agents against acetaminophen-induced hepatotoxicity. 2-(Polyhydroxyalkyl)- and 2-(polyacetoxyalkyl)thiazolidine-4(R)-carboxylic acids.

A directed detoxication mechanism for acetaldehyde (AcH) is described wherein ethanol-derived AcH, circulating in the blood of rats given ethanol-1-14C and disulfiram or pargyline, was sequestered by condensation with administered D(-)-penicillamine (1). The product of this condensation, 2,5,5-trimethylthiazolidine-4-carboxylic acid (3), which was excreted in the urine without acetyl conjugation, was quantitatively determined by inverse isotope dilution measurements. Acetylation of the urine permitted the isolation of the corresponding N-acetyl derivative in crystalline form. The chirality of 3 was deduced by NMR analysis to be 72% 2S, 4S and 28% 2R, 4S. Although acetylation selectively acetylated the predominant isomer, this acetylated derivative was identical in all respects with a chemically synthesized product. This suggests that the in vivo condensation of AcH and 1 is not enzyme mediated.

David J.W. Goon

Papers

Evidence for nitroxyl in the catalase-mediated bioactivation of the alcohol deterrent agent cyanamide

2-Substituted thiazolidine-4(R)-carboxylic acids as prodrugs of L-cysteine. Protection of mice against acetaminophen hepatotoxicity

Prodrugs of L-cysteine as protective agents against acetaminophen-induced hepatotoxicity. 2-(Polyhydroxyalkyl)- and 2-(polyacetoxyalkyl)thiazolidine-4(R)-carboxylic acids.

2,5,5-Trimethylthiazolidine-4-carboxylic acid, a D(-)-penicillamine-directed pseudometabolite of ethanol. Detoxication mechanism for acetaldehyde.

Prodrugs of L-cysteine as liver-protective agents. 2(R,S)-methylthiazolidine-4(R)-carboxylic acid (MTCA), a latent cysteine