Homocysteine, glycine betaine, and N,N-dimethylglycine in patients attending a lipid clinic

Abstract

We recruited nondiabetic subjects (n = 158) attending a lipid disorders clinic, a subset of whom (n = 46) had established cardiovascular disease. Glycine betaine, N,N-dimethylglycine, and carnitine were measured in fasting plasma and urine samples. The concentrations and excretions were related to known cardiovascular risk factors in multivariate regression models. The relationships between homocysteine and plasma and urinary glycine betaine were highly significant (P < .002), comparable with the known relationships with folate and plasma creatinine. The regression coefficient for plasma glycine betaine was consistently approximately −0.1 in 5 different regression models (3 best-subsets and forward and backward stepwise regression models) for predicting homocysteine using 23 variables. Plasma glycine betaine was higher in males than in females, and the difference was associated with a difference in percentage of body fat. Its concentration included a constant factor of approximately 20 μmol/L that was independent of any of the variables investigated here. In the total group, body fat, homocysteine, and carnitine were significant predictors of plasma glycine betaine. Carnitine, an important betaine that is involved in lipid metabolism positively correlated with both homocysteine and glycine betaine. In our sample, the urinary excretion of glycine betaine was outside the reference range in 14 of the 158 subjects and the betaine fractional clearances were above the reference range in 23 subjects. Fractional clearance correlated strongly with plasma homocysteine (r = 0.50), and this relationship may be stronger in patients with known vascular disease. Urinary loss of glycine betaine may contribute to hyperhomocysteinemia and the development of cardiovascular disease.

Introduction

Homocysteine is a nonprotein amino acid that is metabolized via 1 of 2 methionine-conserving methylation pathways, or by catabolism of its carbon skeleton, the first step being conversion to cystathionine (Fig. 1). In most tissues, the methylation of homocysteine is catalyzed by methionine synthase, with N⁵-methyltetrahydrofolate as the methyl donor. This derives from N⁵,N¹⁰-methylenetetrahydrofolate, a reduction catalyzed by the enzyme N⁵,N¹⁰-methylenetetrahydrofolate reductase. The second methylation pathway is catalyzed by the zinc metalloenzyme betaine–homocysteine methyltransferase (BHMT). Glycine betaine is the methyl donor, and the products are methionine and N,N-dimethylglycine. BHMT, found mostly in the liver and kidney (Fig. 1) [1], is subject to feedback inhibition by N,N-dimethylglycine and (to a lesser extent) methionine [2].

Elevations in plasma total homocysteine concentrations have been identified as a risk factor for atherosclerotic disease in the coronary, cerebral, and peripheral vessels and for arterial and venous thrombosis [3], [4], [5], [6], [7]. Factors that cause elevated plasma total homocysteine concentrations include genetic defects [8], [9], [10], certain drugs, renal impairment, age, and nutritional deficiencies. A number of studies have shown an inverse relationship between plasma total homocysteine concentrations and circulating folate [11], [12], vitamin B₆, and vitamin B₁₂ [13], [14], [15].

The role glycine betaine has in the metabolism of homocysteine and its involvement in hyperhomocysteinemia is poorly understood. One reason is that the methods for measuring glycine betaine in human plasma and urine are not widely available. Glycine betaine metabolism is altered in diabetes mellitus [16], [17], [18] and in premature vascular disease [19]. Thus, it seems plausible that perturbations in glycine betaine metabolism could contribute to hyperhomocysteinemia and be associated with the pathophysiology of cardiovascular disease (CVD). To test the plausibility of this hypothesis, we examined a population of patients attending a lipid disorders clinic. We expected that this population would have more patients with multiple cardiovascular risk factors, and hence, would have larger variances than a random population sample. We compared the plasma concentration and urinary excretions of glycine betaine and its metabolites with plasma homocysteine and other known risk factors. Carnitine is also a betaine (Fig. 2) that is transported by many of the same systems that transport glycine betaine [20], [21], [22]. It could affect the activity of BHMT or betaine transport or the expression of their genes. We have previously shown that its metabolism correlates with glycine betaine metabolism in diabetic patients [18]. Therefore, we included carnitine and its main metabolite (acetylcarnitine) in our study.