Folic acid lowers elevated plasma homocysteine in chronic renal insufficiency: possible implications for prevention of vascular disease.

To explore interrelations between folic acid and methionine metabolism in chronic renal insufficiency, we measured plasma amino acids in 21 patients with mean serum creatinine +/- SD of 560 +/- 240 mumol/L, after a ten-hour overnight fast, before and after administration of 5 mg of oral folic acid daily for 15 +/- 6 days. Mean plasma homocysteine was 12.9 +/- 6.8 mumol/L in the patients and 4.2 +/- 0.8 mumol/L in 24 normal controls (P less than .001), and after folic acid administration it declined in the patients to 6.8 +/- 2.8 mumol/L (P less than .0001) in linear proportion (r = .92) to the prefolate homocysteine level. Methionine concentrations were normal in the patients and did not change after folate administration, nor did elevated cysteine and creatinine. Plasma serine was lower (88.3 +/- 17.2 v 121 +/- 25 mumol/L, P less than .41) and declined further to 67.8 +/- 16.4 (P less than .0001) after folate, while prefolate glycine levels increased from 273.3 +/- 61.2 to 313.2 +/- 97.5 mumol/L (P less than .01). Serum and red-cell folate levels were normal in the patients before treatment. The results show that homocysteine levels are increased in chronic renal insufficiency, but may be lowered by folate enhancement of remethylation of homocysteine to methionine. Since elevated plasma homocysteine is associated with premature vascular disease, folic acid may reduce cardiovascular risk in chronic renal insufficiency.

Homocysteinemia: depressed plasma serine levels.

Plasma serine levels were found to be lower than normal (mean +/- SD, 91 +/- 18 mumol/L, n = 16) in homocystinuria patients with a deficiency of cystathionine B-synthase on folate therapy, compared with healthy adults (121 +/- 25 mumol/L, n = 25, P less than 0.001). Of 13 other patients with elevated plasma total homocysteine, two patients with homocystinuria due to remethylation defects had normal serine levels, while 11 renal transplant recipients with mild elevations of serum creatinine had lower than normal serine levels (100 +/- 28 mumol/L, P less than .05). Treatment of both the pyridoxine responsive and nonresponsive cystathionine B-synthase-deficient patients with betaine, which lowered plasma homocysteine, also normalized plasma serine levels. In the two patients with remethylating defects however, betaine lowered plasma homocysteine levels without changing plasma serine levels. By contrast, treatment of the renal transplant patients with pyridoxine, folic acid, and vitamin B12 (cofactors required for homocysteine metabolism), caused falls in plasma homocysteine levels, with a concurrent decline in plasma serine levels. These findings may be explained in terms of the requirements for serine in homocysteine metabolism, both as a source of methyl carbon atoms in the methylation of homocysteine by N5-methyltetrahydrofolate and as a substrate in the cystathionine B-synthase reaction. During periods of elevated plasma total homocysteine in man, increased amounts of serine may be diverted to lowering plasma homocysteine.

Homocystinuria due to cystathionine beta-synthase deficiency--the effects of betaine treatment in pyridoxine-responsive patients.

Homocystinuria due to cystathionine beta-synthase deficiency may be responsive to pyridoxine, a precursor of the cofactor pyridoxal phosphate, and the amount of residual enzyme activity present is the probable determinant of this. In six treated pyridoxine-responsive patients whose biochemical control of fasting plasma amino acid levels appeared optimal, we assessed the effects on plasma amino acids of standard oral methionine loads (4g/m2 of body area) before and after adding betaine (trimethylglycine) 6 g/d, to the treatment regimen of pyridoxine and folic acid. Our aim was to define the capacity of these patients to metabolize methionine and to determine whether betaine would effect a reduction in postload homocysteine levels. During the 24 hours after the methionine challenge all patients had higher plasma methionine and homocysteine and lower cysteine than did 17 normal subjects. After betaine these homocysteine responses were reduced to near normal, and there was a trend toward increased methionine. There was a direct correlation between premethionine fasting homocysteine and mean homocysteine responses during the 24 hours following the methionine load, both before (r = 0.79) and after betaine (r = 0.71). Betaine also increased plasma cysteine levels in patients with the more severe biochemical abnormalities. After betaine there were modest increases in plasma serine (mean increase 25%; P less than 0.025). Since the vascular complications of homocystinuria are related to increased plasma homocysteine, betaine therapy may reduce this risk in patients receiving a standard pyridoxine and folic acid regimen in whom there are abnormal homocysteine responses after a standard methionine load.

Homocystinuria--the effects of betaine in the treatment of patients not responsive to pyridoxine.

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.