Kilmer S. McCully

Homocysteine theory of arteriosclerosis

Arteriosclerotic plaques were found in the aorta and arteries of rabbits given homocysteine thiolactone, methionine or homocysteic acid, both parenterally and in a synthetic diet. Animals given large doses of parenteral methionine or homocysteine thiolactone died of pulmonary embolism and pulmonary infarct. Pyridoxine prevented thrombosis and pulmonary embolism but did not prevent arteriosclerotic plaques. These findings and previous work, showing a new matabolic pathway for sulfate ester synthesis from methionine, the somatotrophic activity of homocysteic acid, and control of cellular growth and intercellular matrix synthesis by homocysteine derivatives, suggest a theory to explain aspects of the pathogenesis of arteriosclerosis.

Homocysteine metabolism and the oxidative modification of proteins and lipids

Altered homocysteine metabolism is implicated as a pathogenic factor in atherogenesis, neoplasia, and aging. Hereditary enzymatic deficiencies and nutritional deficiencies of folate, pyridoxine, or cobalamin are associated with elevated blood homocysteine, accelerated atherosclerosis, and manifestations of aging. The failure of malignant cells to metabolize homocysteine thiolactone to sulfate is attributed to deficiency of thioretinaco, a complex containing cobalamin, homocysteine thiolactone, and retinoic acid. The sulfhydryl group of homocysteine is believed to act catalytically with ferric or cupric ions in a mixed function oxidation system to generate hydrogen peroxide, oxygen radicals, and homocysteinyl radicals. These reactive species may interact with the active site of enzyme protein to cause inactivation of catalytic activity. Homocysteine thiolactone is oxidized to sulfate by a process involving ascorbate, thioretinamide, and superoxide, under the control of thyroxine and growth hormone. Thioretinaco is believed to be the active site of adenosine triphosphate (ATP) binding in oxidative phosphorylation with the participation of oxygen, ascorbate, proton gradient, and electron transport. Depletion of thioretinaco from mitochondrial and microsomal membranes may be associated with increased formation and release of radical oxygen species within neoplastic and senescent cells. Specific proposals are made for investigating the importance of homocysteine metabolism in the oxidative modification of proteins and lipids.

Reduction of plasma lipid and homocysteine levels by pyridoxine, folate, cobalamin, choline, riboflavin, and troxerutin in atherosclerosis☆

Elevated plasma homocysteine and lipid levels are risk factors for atherosclerosis. The plasma levels of homocysteine, determined in acid hydrolyzates of plasma, were found to be correlated with total cholesterol (r = 0.47, P less than 0.001), triglycerides (r = 0.40, P less than 0.01), and body mass index (r = 0.42, P less than 0.01) in 52 males, aged 30-60. A group of 12 male survivors of acute myocardial infarction was given pyridoxine, folate, cobalamin, choline, riboflavin, and troxerutin for 21 days. The plasma concentrations of homocysteine and alpha-amino adipic acid declined to 68% (P less than 0.001) and 57% (P less than 0.001) of the pretreatment values, and the cholesterol, triglycerides, and LDL apo B declined to 79% (P less than 0.001), 68% (P less than 0.01), and 63% (P less than 0.001) of the pretreatment values, respectively. The results suggest a new strategy for control of the metabolic abnormalities in atherosclerosis through the use of naturally occurring, non-toxic nutrients which minimize homocysteine accumulation.

Hyperhomocysteinemia and arteriosclerosis: historical perspectives.

Abstract Early concepts of the origin of arteriosclerosis were introduced in the 19th century by Rokitansky and Virchow, who described mural thrombosis, inflammatory damage to arterial intima, increased intimal permeability to plasma, mucoid degeneration of arterial wall, deposition of plasma lipids in plaques, and fibrosis and calcification of plaques. Experimental production of arteriosclerosis by feeding animal foods to rabbits was attributed to protein intoxication by Ignatowsky in 1908 and to dietary cholesterol by Anitschkow in 1913. Newburgh confirmed the protein hypothesis in 1915–1925 but failed to identify which amino acid produced plaques because methionine (1922) and homocysteine (1932) had not yet been discovered. Cases of homocystinuria from inherited deficiency of cystathionine synthase were found to be associated with thrombosis and vascular disease in 1964. The index case of methionine synthase deficiency (cobalamin C disease) was found by McCully in 1969 to be associated with arteriosclerosis, leading to the homocysteine theory of arteriosclerosis. The theory explains experimental arteriosclerosis by deficiency of vitamin B6 in monkeys, choline deficiency in rats, thyroid deficiency in rats, and methionine deficiency in monkeys. The thrombogenic and atherogenic effects of homocysteine were demonstrated in rabbits, baboons and other species, reproducing the pathological findings found in homocystinuria. Clinical and epidemiological studies in the past two decades have demonstrated that elevated plasma homocysteine is a potent independent risk factor for arteriosclerosis in the general population, supporting the validity of the theory.

Lack of an association or an inverse association between low-density-lipoprotein cholesterol and mortality in the elderly: a systematic review

Objective It is well known that total cholesterol becomes less of a risk factor or not at all for all-cause and cardiovascular (CV) mortality with increasing age, but as little is known as to whether low-density lipoprotein cholesterol (LDL-C), one component of total cholesterol, is associated with mortality in the elderly, we decided to investigate this issue. Setting, participants and outcome measures We sought PubMed for cohort studies, where LDL-C had been investigated as a risk factor for all-cause and/or CV mortality in individuals ≥60 years from the general population. Results We identified 19 cohort studies including 30 cohorts with a total of 68 094 elderly people, where all-cause mortality was recorded in 28 cohorts and CV mortality in 9 cohorts. Inverse association between all-cause mortality and LDL-C was seen in 16 cohorts (in 14 with statistical significance) representing 92% of the number of participants, where this association was recorded. In the rest, no association was found. In two cohorts, CV mortality was highest in the lowest LDL-C quartile and with statistical significance; in seven cohorts, no association was found. Conclusions High LDL-C is inversely associated with mortality in most people over 60 years. This finding is inconsistent with the cholesterol hypothesis (ie, that cholesterol, particularly LDL-C, is inherently atherogenic). Since elderly people with high LDL-C live as long or longer than those with low LDL-C, our analysis provides reason to question the validity of the cholesterol hypothesis. Moreover, our study provides the rationale for a re-evaluation of guidelines recommending pharmacological reduction of LDL-C in the elderly as a component of cardiovascular disease prevention strategies.

Fish oil decreases serum homocysteine in hyperlipemic men

BackgroundBecause of the inverse relation between dietary fish consumption and coronary heart disease and because of the importance of serum homocysteine as an independent risk factor for atherosclerosis, the effect of fish oil on serum homocysteine was studied in hyperlipemic men. MethodsFifteen men with either type IIa or llb lipoproteinemia or hypertriglyceridemia were maintained on a controlled, balanced diet and given either fish oil or olive oil supplements, 12 g/d for 3 weeks, followed by a cross-over period of 3 weeks during which the olive oil or fish oil supplements were given in reverse order. Serum homocysteine was determined by liquid chromatography of acid hydrolyzates of whole serum. ResultsFish oil was found to diminish serum homocysteine levels in 14 of 17 subjects (P<0.01). Serum homocysteine was 48% ±33% less than control values in seven of nine patients and 36% ± 22% less than values in seven of eight subjects who had first received olive oil. There was no effect of olive oil supplements on serum homocysteine, compared with control values, but olive oil produced an increase in serum homocysteine in those who had first received fish oil. Serum triglycerides and very low-density lipoprotein were decreased by fish oil in patients who were first given olive oil, in agreement with previous studies. There was no effect of either fish oil or olive oil on total cholesterol, apolipoprotein B, low-density lipoprotein, or high-density lipoprotein. ConclusionsThe protection against coronary heart disease afforded by a diet rich in fish may be attributed to the lowering of serum homocysteine levels by the n-3 polyunsaturated fatty acids of fish oils.

Homocysteine and the pathogenesis of atherosclerosis

The homocysteine theory of arteriosclerosis was discovered by study of arteriosclerotic plaques occurring in homocystinuria, a disease caused by deficiencies of cystathionine synthase, methionine synthase or methylenetetrahydrofolate reductase. According to the homocysteine theory, metabolic and nutritional abnormalities leading to elevation of plasma homocysteine cause atherosclerosis in the general population without these rare enzymatic abnormalities. Through studies of metabolism of homocysteine thiolactone, the anhydride of homocysteine, in cell cultures from homocystinuric children, the pathway for synthesis of sulfate was found to be dependent upon thioretinamide, the amide formed from retinoic acid and homocysteine thiolactone. Two molecules of thioretinamide form the complex thioretinaco with cobalamin, and oxidative phosphorylation is catalyzed by reduction of oxygen, which is bound to thioretinaco ozonide, by electrons from electron transport particles. Atherogenesis is attributed to formation of aggregates of homocysteinylated lipoproteins with microorganisms, which obstruct the vasa vasorum during formation of arterial vulnerable plaques.

Homocysteine content of lipoproteins in hypercholesterolemia

In order to study the connection between homocysteine and lipid metabolism in atherosclerosis, homocysteine was determined in lipoprotein fractions from men with hypercholesterolemia. All lipoprotein fractions contain a considerably higher level of homocysteine in hypercholesterolemia, compared to normolipemic men, varying from 2.2 to 7.2 times higher estimated per unit volume of serum used for lipoprotein isolation, and from 2.4 to 4.1 times higher, estimated per gram protein. The largest difference in homocysteine content, estimated per gram protein, is present in the LDL fraction, amounting to 4.1 times higher in the hypercholesterolemic than the normolipemic group. In contrast, cholesterol is not higher in hypercholesterolemic than normolipemic men in any lipoprotein fraction, estimated per gram protein, and cholesterol is higher in hypercholesterolemic men only in the LDL fraction, estimated per unit volume. In both LDL and VLDL fractions homocysteine is correlated with cholesterol (r = 0.78, P less than 0.001; r = 0.59, P less than 0.01, respectively) and with protein (r = 0.72, P less than 0.01; r = 0.78, P less than 0.001, respectively). The atherogenic index for homocysteine, LDLHCy/HDLHCy, is 3.5 times higher in the hypercholesterolemic than the normolipemic group. The atherogenic index for cholesterol, LDLChol/HDLChol, is 2.2 times higher in the hypercholesterolemic than the normolipidemic group. The results suggest that analysis of the homocysteine content of the serum and lipoprotein fractions may prove to be useful for assessing risk, prognosis and response to therapy in persons with atherosclerosis.

Homocysteine and lipid metabolism in atherogenesis: effect of the homocysteine thiolactonyl derivatives, thioretinaco and thioretinamide.

In order to study the relation of homocysteine and lipid metabolism to atherogenesis, rabbits were fed a synthetic atherogenic diet and treated with parenteral thioretinaco (N-homocysteine thiolactonyl retinamido cobalamin), thioretinamide (N-homocysteine thiolactonyl retinamide) or homocysteine thiolactone hydrochloride. All three substances were found to increase dietary atherogenesis. Thioretinaco and thioretinamide increase total homocysteine of serum, but there is no effect of parenteral homocysteine thiolactone hydrochloride on serum homocysteine. The synthetic diet with corn oil significantly lowers serum homocysteine, compared either to baseline chow diet or to the synthetic diet with butter. Atherogenesis is correlated with total homocysteine, total cholesterol and LDL + VLDL cholesterol, and serum homocysteine is correlated with total cholesterol, LDL + VLDL, and HDL cholesterol in the total sample. Both synthetic diets elevate serum cholesterol, triglycerides and LDL + VLDL, but not HDL, compared to baseline values. Thioretinamide causes significant elevation of cholesterol and LDL + VLDL, compared to controls. The results show that increased dietary saturated fat and cholesterol cause deposition of lipids within the arteriosclerotic plaques produced by homocysteine, converting fibrous to fibrolipid plaques. Facilitation of atherogenesis is attributed to the effect of homocysteine on artery wall, either from parenteral homocysteine or from the increased synthesis of homocysteine from methionine, produced by thioretinaco and thioretinamide.

Vegetarianism produces subclinical malnutrition, hyperhomocysteinemia and atherogenesis

OBJECTIVE To explain why vegetarian subjects develop morbidity and mortality from cardiovascular diseases unrelated to vitamin B status and Framingham criteria. METHODS A study of 24 rural male subjects 18 to 30 y old and 15 urban male controls was conducted in the Sahel region of Chad. Food consumption was determined from a dietary questionnaire, and overall health status was assessed by body weight, body mass index, serum albumin, plasma transthyretin, urinary nitrogen, and creatinine. Plasma lipids, vitamins B6, B9 and B12, homocysteine, and related sulfur amino acids were measured as selected cardiovascular disease risk factors. RESULTS Body weight, body mass index, blood, and urinary markers of protein status were significantly lower, with an estimated 10% decrease of lean body mass in the study group compared with urban controls. Neither lipid fractions nor plasma levels of vitamins B6, B9, and B12 were significantly different between the two groups. Although the mean consumption of sulfur amino acids (10.4 mg·kg(-1)·d(-1)) by rural subjects was significantly below the recommended dietary allowances (13 mg·kg(-1)·d(-1)), plasma methionine values were similar in the two groups. In contrast, homocysteine concentration was significantly increased (18.6 μmol/L, P < 0.001), and the levels of cysteine and glutathione were significantly decreased in the study group, demonstrating inhibition of the trans-sulfuration pathway. The strong negative correlation (r = -0.71) between transthyretin and homocysteine implicated lean body mass as a critical determinant of hyperhomocysteinemia. CONCLUSION The low dietary intake of protein and sulfur amino acids by a plant-eating population leads to subclinical protein malnutrition, explaining the origin of hyperhomocysteinemia and the increased vulnerability of these vegetarian subjects to cardiovascular diseases.