The effect of D-penicillamine on protein-bound homocyst(e)ine in homocystinurics

Hydrolysis of homocysteine thiolactone results in the formation of Protein-Cys-S-S-homocysteinylation

An increased level of homocysteine, a reactive thiol amino acid, is associated with several complex disorders and is an independent risk factor for cardiovascular disease. A majority (>80%) of circulating homocysteine is protein bound. Homocysteine exclusively binds to protein cysteine residues via thiol disulfide exchange reaction, the mechanism of which has been reported. In contrast, homocysteine thiolactone, the cyclic thioester of homocysteine, is believed to exclusively bind to the primary amine group of lysine residue leading to N-homocysteinylation of proteins and hence studies on binding of homocysteine thiolactone to proteins thus far have only focused on N-homocysteinylation. Although it is known that homocysteine thiolactone can hydrolyze to homocysteine at physiological pH, surprisingly the extent of S-homocysteinylation during the exposure of homocysteine thiolactone with proteins has never been looked into. In this study, we clearly show that the hydrolysis of homocysteine thiolactone is pH dependent, and at physiological pH, 1 mM homocysteine thiolactone is hydrolysed to ~0.71 mM homocysteine within 24 h. Using albumin, we also show that incubation of HTL with albumin leads to a greater proportion of S-homocysteinylation (0.41 mol/mol of albumin) than N-homocysteinylation (0.14 mol/mol of albumin). S-homocysteinylation at Cys³⁴ of HSA on treatment with homocysteine thiolactone was confirmed using LC-MS. Further, contrary to earlier reports, our results indicate that there is no cross talk between the cysteine attached to Cys³⁴ of albumin and homocysteine attached to lysine residues.

Hyperhomocysteinemia and thrombophilia

It is now widely accepted that hyperhomocysteinemia (HHC) is a risk factor for thrombophilia. HHC is the result of either impaired enzyme function or a deficiency of vitamin B (folate, B₆, B₁₂), or both, and can be treated with vitamin supplements. Measuring plasma total homocysteine (tHcy) is included in the routine thrombophilia panel in many laboratories, despite having a limited value to the clinician. Many methods are available for tHcy measurements. High-pressure liquid chromatography (HPLC) with fluorescence detection is a widely used method, but is being replaced by more convenient immuno- or enzyme assays. In this paper a general overview on homocysteine is given, with an emphasis on laboratory methods.

Plasma d-penicillamine redox state evaluation by capillary electrophoresis with laser-induced fluorescence

D-Penicillamine (D-Pen) is a thiol drug used in the treatment of Wilson's disease, rheumatoid arthritis, metal intoxication and cystinuria. We have recently described a new capillary electrophoresis (CE) method to measure physiological thiols, in which separation of total plasma homocysteine, cysteine, cysteinylglycine, glutathione is achieved using the organic base N-methyl-D-glucamine in the run buffer. In this paper, we present an improvement of our method that allows a baseline separation of total plasma D-Pen from the physiological thiols. Moreover, reduced, free and protein-bound forms of drug are measured by varying the order of disulfide reduction with tributylphosphine and proteins precipitation with 5-sulphosalicylic acid (SSA). After derivatization with 5-iodoacetamidofluorescein (5-IAF), samples are separated and measured by capillary electrophoresis with laser-induced fluorescence in an uncoated fused-silica capillary (57 x 75 microm i.d.) using a phosphate/borate run buffer pH 11.4. In these conditions, the migration time of D-Pen is about 7 min and the time required for each analysis is roughly 10 min. The proposed method has been utilized to measure the various forms of the drug in a D-Pen administered Wilson's disease patient.

Facts and Recommendations about Total Homocysteine Determinations: An Expert Opinion

Background: Measurement of plasma total homocysteine has become common as new methods have been introduced. A wide range of disorders are associated with increased concentrations of total homocysteine. The purpose of this review is to provide an international expert opinion on the practical aspects of total homocysteine determinations in clinical practice and in the research setting and on the relevance of total homocysteine measurements as diagnostic or screening tests in several target populations.

Methods: Published data available on Medline were used as the basis for the recommendations. Drafts of the recommendations were critically discussed at meetings over a period of 3 years.

Outcome: This review is divided into two sections: (a) determination of homocysteine (methods and their performance, sample collection and handling, biological determinants, reference intervals, within-person variability, and methionine loading test); and (b) risk assessment and disease diagnosis (homocystinuria, folate and cobalamin deficiencies, cardiovascular disease, renal failure, psychiatric disorders and cognitive impairment, pregnancy complications and birth defects, and screening of elderly and newborns). Each of these subsections concludes with a separate series of recommendations to assist the clinician and the research scientist in making informed decisions. The review concludes with a list of unresolved questions.

Plasma reduced homocysteine concentrations are increased in end-stage renal disease

BACKGROUND

Plasma total homocysteine (tHcy) concentrations> 15 micromol/L are associated with an increased risk of cardiovascular disease. This is especially the case in end-stage renal disease (ESRD), in which tHcy concentrations commonly range between 20 and 30 micromol/L. Adverse vascular or prothrombotic effects associated with hyperhomocysteinemia are assumed to be mediated by the free sulfhydryl (reduced) form of the molecule (rHcy), but data based on fluorescence high-pressure liquid chromatography (HPLC) indicate that rHcy concentrations are not increased in ESRD despite two- to threefold elevations in tHcy.

METHODS

We developed a sensitive method for measuring plasma rHcy concentrations in which freshly drawn blood is incubated with sodium iodoacetate, and the resulting S-carboxymethylhomocysteine is analyzed by gas chromatography mass spectrometry.

RESULTS

Unlike with the earlier methodology, we found plasma rHcy concentrations two to four times higher than normal in ESRD. These concentrations were lowered by hemodialysis and were proportional to plasma tHcy over the range of tHcy concentrations that has been associated with increased cardiovascular risk (r2 = 0.39, P < 0.0001).

CONCLUSIONS

These results support the hypothesis that homocysteine could directly mediate vascular disease through mechanisms related to the reactivity of its free sulfhydryl group. It remains to be determined how much of the variability between plasma tHcy and rHcy is due to analytical variation and how much is due to biologic factors that separately influence concentrations of the disease marker, tHcy, and its presumed mediator, rHcy.

Mechanisms for the formation of protein-bound homocysteine in human plasma

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Greater than 70% of homocysteine in circulation is protein-bound. An in vitro model system using human plasma has been developed to study mechanisms of protein-bound homocysteine formation and establish the equilibrium binding capacities of plasma for homocysteine. Addition of homocysteine to plasma caused an initial rapid displacement of cysteine and a subsequent increase in protein-bound homocysteine. This rapid reaction was followed by a slower oxygen-dependent reaction forming additional protein-bound homocysteine. To determine the equilibrium binding capacity of plasma proteins for homocysteine, plasma was treated with 0.5-10 mM dl-homocysteine for 4 h at 37 degrees C under aerobic conditions. Under these conditions the equilibrium binding capacity was 4.88 +/- 0.51 and 4.74 +/- 0.68 micromol/g protein for male (n = 10) and female (n = 10) donors, respectively. The mechanism of protein-bound homocysteine formation involves both thiol-disulfide exchange and thiol oxidation reactions. We conclude that plasma proteins have a high capacity for binding homocysteine in vitro.

Nutritional aspects and possible pathological mechanisms of hyperhomocysteinaemia: an independent risk factor for vascular disease

Numerous case-control and prospective studies have identified elevated plasma homocysteine as a strong independent risk factor for cerebovascular, cardiovascular and peripheral vascular disease. Homocysteine is formed as a result of the breakdown of the dietary amino acid methionine. Once formed, homocysteine is either remethylated to methionine, or undergoes a trans-sulfuration reaction to form cysteine. The re-methylation of homocysteine to methionine is dependent on three B-vitamins, i.e. riboflavin, vitamin B12 and folate. The second pathway of homocysteine metabolism is the trans-sulfuration pathway which requires both vitamin B6 and riboflavin for its activity. Thus, up to four B-vitamins are required for intracellular homocysteine metabolism. Many studies have noted strong inverse relationships between homocysteine levels and the status of both vitamin B12 and folate. However, the relationship between vitamin B6 status and homocysteine is still uncertain. Similarly, numerous intervention studies have demonstrated effective lowering of homocysteine levels as a result of folate and vitamin B12 supplementation, while the homocysteine-lowering ability of vitamin B6 is unclear. Even though riboflavin plays a crucial role in both the trans-sulfuration and remethylation pathways of homocysteine metabolism, the relationship between riboflavin status and homocysteine levels has not been investigated. The exact mechanism that explains the vascular toxicity of elevated homocysteine levels is unknown at present, studies indicate that it is both atherogenic and thrombogenic. To date, no randomized clinical trial has demonstrated that lowering of homocysteine levels is beneficial in terms of reducing the prevalence of vascular disease. It is probable, however, that optimal B-vitamin status is important in the prevention of vascular disease.

Niacin treatment increases plasma homocyst(e)ine levels

BACKGROUND

Studies have reported high levels of plasma homocyst(e)ine as an independent risk factor for arterial occlusive disease. The Cholesterol Lowering Atherosclerosis Study reported an increase in plasma homocyst(e)ine levels in patients receiving both colestipol and niacin compared with placebo. Thus the objective of this study was to examine the effect of niacin treatment on plasma homocyst(e)ine levels.

METHODS

The Arterial Disease Multiple Intervention Trial, a multicenter randomized, placebo-controlled trial, examined the effect of niacin compared with placebo on homocyst(e)ine in a subset of 52 participants with peripheral arterial disease.

RESULTS

During the screening phase, titration of niacin dose from 100 mg to 1000 mg daily resulted in a 17% increase in mean plasma homocyst(e)ine level from 13.1 +/- 4.4 micromol/L to 15.3 +/- 5.6 micromol/L (P <.0001). At 18 weeks after randomization, there was an absolute 55% increase from baseline in mean plasma homocyst(e)ine levels in the niacin group and a 7% decrease in the placebo group (P =.0001). This difference remained statistically significant at the end of follow-up at 48 weeks.

CONCLUSIONS

Niacin substantially increased plasma homocyst(e)ine levels, which could potentially reduce the expected benefits of niacin associated with lipoprotein modification. However, plasma homocyst(e)ine levels can be decreased by folic acid supplementation. Thus further studies are needed to determine whether B vitamin supplementation to patients undergoing long-term niacin treatment would be beneficial.

A theory for the mechanism of homocysteine-induced vascular pathogenesis

An attempt is made to reveal the mechanism of homocysteine-induced vascular pathogenesis and a theory is developed on the basis of an analysis of available data. In the blood, homocysteine molecules, which possess the capacity of forming chelate complexes with metallic cations including calcium ions, interact with the calcium ions of the calcium-dependent cell adhesions/junctions of the vascular endothelium. Such an action results in the departure of calcium ions from some cell adhesions/junctions, causing the latter structures to dissociate and the vascular endothelium to be injured. While such factors as high levels of plasma calcium ion tend to restore the disrupted cell adhesions/junctions, other factors including blood pressure, bloodstream shearing force and high cholesterol/phospholipid ratio in the membranes are unfavorable to the repair. The injuries to the vascular endothelium may initiate inflammatory reactions. Over a long time, as such endothelial injuries occur repeatedly and, additionally, a small number of unrepaired cell adhesion/junction disruptions exacerbate slowly but progressively, inflammation may be constantly present. As the inflammation becomes excessive or uncontrollable, irreversible vascular pathogenesis takes place. Added to the condition is thrombosis caused by excessive coagulation activities triggered by the vascular endothelium injuries. In addition to the interference due to decreased membrane fluidity to the restoration of interrupted cell adhesions/junctions, cholesterol may further promote vascular pathogenesis (induced by either homocysteine or other factors) by impeding the detachment and departure of the foam cells from the vascular lesions. Such impediment is presumably related to an inhibitory effect on cell shape change and cell movement by the formation of a stable and rigid cytoskeleton-membrane network characterized by reduced membrane malleability and enhanced cytoskeleton-membrane association.

Total homocysteine and cardiovascular disease

Recent data have shown that an elevated plasma level of the amino acid homocysteine (Hcy) is a common, independent, easily modifiable and possibly causal risk factor for cardiovascular disease (CVD) which may be of equal importance to hypercholesterolemia, hypertension and smoking. This paper reviews the biochemical, clinical, epidemiological and experimental data underlying this conclusion and is critically questioning whether elevated tHcy is a causal factor.

Regulation of homocysteine metabolism

We have used a combination of in vivo and in vitro techniques to measure factors regulating homocysteine metabolism and the plasma concentration of this atherogenic amino acid. The germane findings include: 1. Homocysteine metabolism in rat kidney proceeds predominantly through the transsulfuration pathway, whose enzymes are enriched within the proximal cells of kidney tubules. Furthermore, the rat kidney possesses significant reserve capacity to handle both acute and chronic elevations in plasma homocysteine concentrations. 2. Plasma homocysteine concentrations are lower in diabetic rats. Insulin administration corrects this perturbation. Therefore, insulin and/or one of its counter-regulatory hormones affects homocysteine metabolism, possibly through an increased flux in the hepatic transsulfuration pathway. In support of these data, glucagon administration to rats produced similar results. Further support was provided by studies with isolated rat hepatocytes, from which homocysteine export was reduced when incubated in the presence of glucagon.

Homocysteine: cholesterol of the 90s?

Homocysteine is a sulfur-containing amino acid generated through the demethylation of methionine. It is largely catabolized by trans-sulfuration to cysteine but it may also be remethylated to methionine. Dubbed 'the cholesterol of the 90s' by the lay press, homocysteine is thought to be thrombophilic and to damage the vascular endothelium. Total plasma homocysteine (tHcy) is now established as a clinical risk factor for coronary artery disease, as well as other arterial and venous occlusive disease in adult populations. Regulation of homocysteine is dependent on nutrient intake, especially folate, vitamins B6 and B12. It is also controlled by common genetic variations (polymorphisms) in how vitamins are utilized as cofactors in the reactions controlling homocysteine metabolism. Moreover, concentrations are age- and sex-dependent and are altered by renal function, hormonal status, drug intake and a variety of other common clinical factors. Considerable care must be taken in assaying tHcy. Plasma should be separated shortly after collection, to avoid artifactual increases due to synthesis by blood cells in vitro. Reference methods have not been validated and criteria for establishing reference ranges should take into account the variable prevalence of physiological hyperhomocysteinemia. Determination of tHcy should probably be limited to centres with relevant expertise and ability to maintain the high degree of precision required for reliable interpretation. Molecular testing for the genetic polymorphisms is still in the research phase but the ease and reliability of molecular diagnosis will speed its introduction into clinical laboratory practice--particularly in relation to diagnosis of thrombophilic disorders. Clinical research initiatives are being driven by the benefit that should be achieved by correction with vitamin supplements, particularly folate and B vitamins, but it must be recognized that prospective controlled studies to validate clinical benefit are only now being initiated. At the moment, it is safe to say that hyperhomocysteinemia is one of the few prevalent biochemical risk factors for thromboembolic disease that might be corrected by vitamin supplements. Such a possibility lies behind the growing momentum to recommend increased supplements of folate and B vitamins to at-risk population and patient groups today.

Homocysteine

Homocysteine is a sulfur-containing amino acid generated through the demethylation of methionine. It is largely catabolized by trans-sulfuration to cysteine, but it may also be remethylated to methionine. Regulation of homocysteine is dependent on nutrient intake, especially folate, vitamins B6 and B12. It is also controlled by individual genetic differences in how vitamins are utilized as cofactors in the reactions controlling homocysteine metabolism. In excess quantities, homocysteine is thought to be thrombophilic and to damage the vascular endothelium. Total plasma homocysteine (tHcy) is now established as a clinical risk factor for coronary artery disease, as well as other arterial and venous occlusive disease in adult populations. These effects are probably related to its role as a teratogen in the pathogenesis of neural tube defects--genetic variants causing hyperhomocysteinemia are associated with both neural tube defects in susceptible pregnancies and with risks for vaso-occlusive disease in later years. Considerable care must be taken in assaying tHcy. Plasma should be separated shortly after collection to avoid artifactual increases due to synthesis by blood cells in vitro. tHcy concentrations must be interpreted in light of the fact that serum albumin, urate, creatinine, and vitamin concentrations may be important analytical covariates. Moreover, concentrations are age- and sex-dependent and are altered by renal function, hormonal status, drug intake, and a variety of other common clinical factors. Why then is homocysteine now of such great clinical and scientific interest? If the homocysteine moiety itself is important in the pathogenesis of vaso-occlusive disease, then simple treatment of hyperhomocysteinemia with vitamins should lead to a significant reduction in disease risk. Such a possibility lies behind the growing momentum to recommend increased supplements of folate and B vitamins to at-risk populations and patient groups today.

Determination of plasma and serum homocysteine by high-performance liquid chromatography with fluorescence detection

Determination of homocysteine in plasma or serum is becoming an important diagnostic procedure. Accurate, rapid and low cost methods for measuring homocysteine are therefore required. We have improved an HPLC method and made it suitable for clinical application. The total homocysteine in plasma consists of free homocysteine (i.e., reduced plus oxidized homocysteine in the non-protein fraction of plasma) and protein-bound homocysteine. The method consists of the following steps: reduction of the sample with tri-n-butylphosphine, precipitation of proteins with trichloroacetic acid (10%) and derivatization with ammonium 7-fluorobenzo-2-oxa-1,3-diazole-4-sulfonate. The derivatives are separated by reversed-phase high-performance liquid chromatography followed by fluorescence detection. The concentrations (mean +/- S.D.) of total homocysteine in plasma from 77 normal subjects, 44 male and 33 female adults, were 8.4 +/- 2.15 and 7.1 +/- 1.18 mumol/l, respectively. Serum concentrations were 8.8 +/- 2.6 mumol/l in males and 7.6 +/- 1.5 mumol/l in females.

Renal uptake and excretion of homocysteine in rats with acute hyperhomocysteinemia

BACKGROUND

Elevated plasma total homocysteine, an independent risk factor for cardiovascular disease, is commonly observed in renal patients. We have previously shown that the kidney is a major site for the removal of plasma homocysteine in the rat. The present investigation was performed to further characterize the capacity of the kidney to handle acute elevations in plasma homocysteine concentrations.

METHODS

Acute hyperhomocysteinemic conditions (4- to 7-fold > controls) in rats were produced by either a primed-continuous infusion of L-homocysteine or exposure to 80:20% nitrous oxide:oxygen, which results in the inhibition of methionine synthase.

RESULTS

At physiological homocysteine concentrations, approximately 15% of the arterial plasma homocysteine was removed on passage through the kidney. Renal homocysteine uptake was approximately 85% of the filtered load. The urinary excretion of homocysteine was negligible (<2%). During acute hyperhomocysteinemia produced by the infusion of L-homocysteine, renal homocysteine uptake was increased fourfold and was equivalent to 50% of the infused dose, while urinary excretion remained negligible. Renal homocysteine uptake during nitrous oxide-induced hyperhomocysteinemia increased threefold, with urinary excretion remaining negligible.

CONCLUSIONS

These results provide strong evidence that the kidney has a significant capacity for metabolizing acute elevations in plasma homocysteine, and support a very limited role for the re-methylation pathway in renal homocysteine metabolism.

Enzyme conversion immunoassay for determining total homocysteine in plasma or serum

A rapid and precise immunoassay for quantification of total homocysteine in blood samples is presented. The method avoids the use of radioisotopes and chromatographic separations and relies on enzymatic conversion of homocysteine to S-adenosyl-l-homocysteine, followed by quantification of S-adenosyl-l-homocysteine by an enzyme-linked immunoassay in microtiter format. The within- and between-assay imprecision is &lt;6% and 8%, respectively, and results by the method show good correlation with those by HPLC. Including controls and calibrators in duplicates, 82 samples can be analyzed within 2.5 h. The procedure can be fully automated.

Folate and homocysteine status and haemolysis in patients treated with sulphasalazine for arthritis

In an attempt to estimate the frequency of folate deficiency and haemolysis in a group of 25 outpatients with arthritis treated with sulphasalazine (SASP), haematological measurements, including plasma total homocysteine (tHcy) which is a sensitive marker of folate deficiency, serum folate (S-folate), erythrocyte (RBC) folate, S-cobalamin and routine indices of haemolysis were performed. No patient had been taking folate-containing vitamins for at least 8 weeks prior to the study. Compared to a group of 72 healthy hospital staff, the median plasma tHcy was significantly higher in the patient group (8.8 mumol 1(-1) vs. 6.8 mumol 1(-1); p = 0.003). Five patients (20%) had plasma tHcy levels that exceeded the upper normal limit of plasma tHcy (median+2 SD of the reference group). Median S-folate was significantly lower in the patient group (6.0 nmol 1(-1) vs. 8.5 nmol 1(-1); p < 0.001), and 11 (44%) patients had depressed S-folate. Only three (12%) patients had RBC folate values below the reference interval. There was no difference in the levels of RBC folate between the two groups. A comparison of S-cobalamin levels in the two groups disclosed a significantly lower level in the patient group. However, no patient had cobalamin deficiency as assessed by S-cobalamin and S-methylmalonate measurements. Thus, it is unlikely that any patient had increased plasma tHcy due to cobalamin deficiency. Of 24 patients having a HbA1c measurement performed, 12 (50%) had decreased levels indicating chronic haemolysis. Only seven (28%) patients had reticulocytosis. HbA1c was positively correlated to haptoglobin levels (r = 0.77; p < 0.001) and negatively correlated to the percentage of reticulocytes (r = -0.50; p = 0.02). The percentage of reticulocytes was negatively correlated to haptoglobin levels (r = -0.42; p = 0.04). The chronic haemolysis of the patients' blood due to SASP might explain the similar RBC folate values in the two groups because of a relatively higher folate content of young erythrocytes. In conclusion, our results support previous findings of folate deficiency and haemolysis occurring in a considerable fraction of patients receiving treatment with SASP. Measurements of plasma tHcy suggest that a substantial number of patients may have folate deficiency at the tissue level.

Homocyst(e)ine and arterial occlusive diseases

Homocysteine is a thiol-containing amino acid resulting from demethylation of methionine. The free and protein-bound forms of the amino acid and derived disulfides are called homocyst(e)ine [H(e)]. Multiple studies have shown elevated H(e) levels in patients with coronary, cerebrovascular, or peripheral arterial diseases; this association is frequent and independent of most other risk factors for atherosclerosis. In the 1993 Frontiers in Medicine Symposium investigators discussed the genetic, physiological, nutritional, and pharmacological mechanisms involved in the regulation of plasma H(e), the association of H(e) with arterial occlusive diseases, and the relationships of H(e) with nitric oxide and haemostasis. High plasma H(e) levels usually can be reversed with vitamin supplements. Whether vitamin supplements will affect the evolution of arterial occlusive diseases needs to be established in prospective, placebo-controlled, randomized, clinical trials.

Plasma homocysteine and thiol compound fractions after oral administration of N-acetylcysteine

The total concentration of the atherogenic aminothiol acid homocysteine in plasma of healthy volunteers was decreased after oral administration of N-acetylcysteine (NAC), whereas the reduced and free (non-protein bound) fractions of homocysteine were increased. The decrease of the total fraction varied between 20 and 50% and was dose-related. Cysteinylglycine was also decreased after the administration of NAC, whereas cysteine did not change. Administration of high amounts of NAC probably displaces homocysteine and cysteinylglycine from their protein binding sites by disulfide interchange reactions. This leads to the formation of mixed low molecular-weight cysteine and NAC disulfides with high renal clearance and possibly also increased metabolic bio-availability, thereby eliminating homocysteine and cysteinylglycine from plasma. Since only a small amount of additional urinary homocysteine was recovered it is likely that this aminothiol acid is taken up by the tubular cells and further metabolized.

Clinical significance of pharmacological modulation of homocysteine metabolism

The metabolic fate of homocysteine is linked to vitamin B12, reduced folates, vitamin B6 and sulfur amino acids. Clinical and experimental data suggest that elevated plasma homocysteine is an independent risk factor for premature vascular disease. This is particularly significant because plasma homocysteine levels are altered in several diseases, including folate and vitamin B12 deficiencies, and because many commonly used drugs have now been shown to interfere with homocysteine metabolism. In summarizing the data, Helga Refsum and Per Ueland highlight the clinical implications for these metabolic changes.

Determination of homocysteine, penicillamine, and their symmetrical and mixed disulfides by liquid chromatography with electrochemical detection

A method for the determination of D-penicillamine, homocysteine, homocystine, penicillamine-homocysteine mixed disulfide, and penicillamine disulfide in human plasma and urine is described. The method involves separation of the various thiols and disulfides by high-performance liquid chromatography with detection by a dual Hg/Au amalgam electrochemical detector. D-Penicillamine and homocysteine are detected at the downstream electrode; the disulfides are first reduced to thiols at the upstream electrode and then the thiols are detected at the downstream electrode. Hydrodynamic voltammograms were measured for the various thiols and disulfides to determine optimum settings for the electrochemical detector, and the effect of mobile phase parameters on retention times was studied to optimize the separation. A convenient method for the preparation of calibration solutions of penicillamine-homocysteine mixed disulfide by thiol/disulfide exchange with standardization of the solution by H NMR spectroscopy is described. Detection limits are below the concentrations of homocystine and penicillamine-homocysteine mixed disulfide reported to be present in the plasma and urine, respectively, of homocystinuric patients under treatment with D-penicillamine.

Homocysteinemia in rats induced by folic acid deficiency

The effect of folate deficiency on homocysteine metabolism was examined in rats given a folate-deficient diet. Total homocysteine was determined in serum stored at -22 degrees C for 3 wk. All animals in the control group had more than 20 ng.ml-1 of serum folate and more than 1000 pg.ml-1 of serum cyanocobalamin throughout the experimental period. In contrast, serum folate in animals given the folate-deficient diet decreased to less than 3 ng.ml-1 after 4 wk and to less than 2 ng.ml-1 (a subnormal level) after 10 wk of the experiment while serum cyanocobalamin remained at more than 1000 pg.ml-1 throughout the experiment. In the control group, mean serum total homocysteine +/- SD was 4.04 +/- 1.07 nmol.ml-1 during the 20 wk of experiment. At the 10th wk before serum folate reached subnormal levels, the animals given the folate-deficient diet had a mean serum total homocysteine of 7.67 +/- 1.53 nmol.ml-1, demonstrating a significant increase (P less than 0.001). No further significant increase of mean serum total homocysteine concentrations was observed after serum folate became subnormal. This study demonstrated for the first time that a selective deficiency of folic acid caused a 2-4 fold increase in serum total homocysteine when serum folate was at low normal and at subnormal levels in rats.

Interrelations between plasma free and protein-bound homocysteine and cysteine in homocystinuria

To study the interrelations between plasma free and protein-bound homocysteine and cysteine, we measured the levels of these four variables in 167 samples from 17 patients with homocystinuria during different treatment regimens, 14 heterozygotes for cystathionine B-synthase deficiency, and 17 normal subjects. There was a strong positive correlation between free and protein-bound homocysteine, and between free and protein-bound cysteine over a wide range of values in varying clinical situations, but homocysteine and cysteine had differing binding characteristics. At low concentrations homocysteine bound more tightly than cysteine to plasma protein, while at high concentrations of the free amino acids more cysteine than homocysteine was bound to plasma protein. In patients with homocystinuria due to cystathionine B-synthase deficiency, levels of protein-bound homocysteine after an overnight fast were eight to 12 times higher than mean levels +/- SD in the normal subjects of 0.15 +/- 0.03 mumol/g of plasma protein, n = 17, and levels of protein-bound cysteine were lower than mean normal levels +/- SD of 2.30 +/- 0.23 mumol/g, n = 17. In the patients before treatment the proportions of both plasma thiols that were protein-bound were approximately half those in the normal subjects. For homocysteine the proportion was 35% in a representative patient and 78% in normal subjects and in heterozygotes, while for cysteine the corresponding proportions were 26% and 59%. In all blood samples the sum of the plasma free and protein-bound homocysteine and cysteine remained relatively constant (mean +/- SD = 270.6 +/- 68.6 mumol/L of plasma, n = 142).(ABSTRACT TRUNCATED AT 250 WORDS)

Homocysteinemia due to folate deficiency

We examined the relationship between serum folate and total homocyst(e)ine levels by determining protein-bound homocyst(e)ine in stored serum from 19 subjects with subnormal serum folate (less than 2 ng/mL), 137 subjects with low normal serum folate (between 2.0 and 3.9 ng/mL), 44 subjects with normal serum folate (between 4.0 and 17.9 ng/mL), and 38 subjects with high serum folate (above 18 ng/mL). Eighty-four percent of the subjects with subnormal serum folate and 56% of the subjects with low normal serum folate had more than 7.05 nmol/mL serum total homocyst(e)ine (ie, more than two standard deviations above the normal mean). Thirty-two percent of these subjects had more than a three-fold increase in serum total homocyst(e)ine. These observations support the hypothesis that depletion of tissue folate causes homocysteinemia in nonhomocystinuric subjects. Subnormal as well as low normal concentrations of serum folate appear to produce an accumulation of homocyst(e)ine. In addition, relatively normal levels of serum total homocyst(e)ine were observed in four pregnant women with low serum folate, supporting previous suggestions of an influence of female sex hormone(s) in homocysteine metabolism.

Protein-bound homocyst(e)ine in patients with rheumatoid arthritis undergoing D-penicillamine treatment

Protein-bound homocyst(e)ine was measured in the plasma of 38 nonhomocystinuric patients with rheumatoid arthritis. Nineteen of them were treated orally with D-penicillamine 100-1,500 mg/d for a period of one month to 15 years. For these patients, the mean +/- standard deviation level of plasma protein-bound homocyst(e)ine was 1.95 +/- 1.07 nmol/mL. In contrast, the mean plasma level of protein-bound homocyst(e)ine was 4.72 +/- 1.11 nmol/mL in the 19 patients who had not been treated with oral D-penicillamine. There was a statistically significant difference (P less than .0001) in the plasma protein-bound homocyst(e)ine concentrations between patients with and without oral D-penicillamine therapy. Thus, it may be speculated that oral D-penicillamine may be beneficial in protecting patients from the development of thromboembolism and arteriosclerosis.

Total homocyst(e)ine in plasma and amniotic fluid of pregnant women

Total homocyst(e)ine was determined by the quantitation of protein-bound homocyst(e)ine in the stored plasma and amniotic fluid from 25 pregnant women and in the stored plasma from 17 nonpregnant women. The mean +/- SE of plasma total homocyst(e)ine was 29.8 +/- 2.4 nmol/g protein in pregnant women and 52.4 +/- 3.8 nmol/g protein in nonpregnant women. In contrast, the mean +/- SE of total homocyst(e)ine in amniotic fluid obtained at 16 weeks of gestation was 36.3 +/- 2.9 nmol/g protein. There was a statistically significant difference in the plasma total homocyst(e)ine concentrations from pregnant and nonpregnant women (P less than 0.01). Similarly, there was also a statistically significant difference between plasma total homocyst(e)ine from nonpregnant women and amniotic fluid total homocyst(e)ine (P less than 0.01). These observations suggested that the metabolism of homocysteine to cysteine was more efficient in pregnant women. In addition, the concentrations of total homocyst(e)ine in amniotic fluids were within narrow limits in normal pregnancies. Hence, total homocyst(e)ine concentration might be very valuable as a rapid assessment of fetuses for congenital defects of homocysteine metabolism.

Protein-bound homocyst(e)ine. A possible risk factor for coronary artery disease

The development of atherosclerotic changes and thromboembolism are common features in homocystinurics. Hence, we postulate a positive correlation between the level of homocyst(e)ine in the blood and the occurrence of coronary artery disease. Homocysteine is found either as free homocystine, cysteine-homocysteine mixed disulfide, or protein-bound homocyst(e)ine. In nonhomocystinuric subjects, most homocysteine molecules are detectable in the protein-bound form. Thus, protein-bound homocyst(e)ine in stored plasma which reflected total plasma homocyst(e)ine was determined in 241 patients with coronary artery disease (173 males and 68 females). The mean +/- SD total plasma homocyst(e)ine was 5.41 +/- 1.62 nmol/ml in male patients, 4.37 +/- 1.09 nmol/ml in male controls, 5.66 +/- 1.93 nmol/ml in female patients, and 4.16 +/- 1.62 nmol/ml in female controls. The differences between the patients with coronary artery disease and the controls were statistically significant (P less than 0.0005).