Oral Methionine Load Research Articles

Effect of methionine loading and endogenous hypermethioninaemia on blood mercaptans in man

In five patients with cirrhosis given an oral methionine load, blood mercaptan concentrations were not significantly affected by neomycin and metronidazole therapy. Methanethiol and dimethyl sulphide rose after methionine to levels encountered in hepatic encephalopathy but in stable cirrhotics no neurological abnormalities were evident. In one patient with chronic hepatic encephalopathy there was no significant change in methanethiol, dimethyl sulphide or ammonia concentration 4 h after methionine when conscious state had deteriorated by two stages of encephalopathy. Elevations in blood methanethiol and dimethyl sulphide concentration in children with congenital hypermethioninaemia were not associated with any neurological or electroencephalographic features of hepatic coma. These studies do not support an important role for mercaptans in hepatic encephalopathy although a synergistic effect cannot be excluded. Furthermore, mercaptans appear to arise from endogenous metabolism rather than bacterial action in the gut.

Improved identification of heterozygotes for homocystinuria due to cystathionine synthase deficiency by the combination of methionine loading and enzyme determination in cultured fibroblasts

Previous data on tentative identification of the carrier state for homocystinuria due to cystathionine synthase deficiency using methionine loading or measurement of cystathionine synthase activity in tissue extracts are conflicting. We studied the results of standardized oral methionine loading in 20 obligate heterozygotes and compared them with those of determination of cystathionine synthase activity in cultured fibroblasts. Special attention was devoted to our recently reported observation on the small but striking differences in methionine metabolism between healthy pre- and postmenopausal women and men. Fasting and after load peak levels of methionine in serum did not discriminate the carriers from the control subjects. The mean fasting level of total homocysteine was only significantly higher in the group of premenopausal heterozygotes than in the corresponding control group. Nevertheless, the individual values overlapped with the normal range in 4 of 12 premenopausal heterozygotes. After loading peak levels of total homocysteine in 18 out of the 20 obligate heterozygotes exceeded the upper limit of the ranges in the three control groups. Thus, this parameter discriminated 90% of the obligate carriers. Measurement of cystathionine synthase activity in cultured fibroblasts from a skin biopsy identified the obligate heterozygotes to a similar degree (85%). No significant correlation between the measurements of cystathionine synthase activity and the after load peak levels of total homocysteine in the individual heterozygotes was established. Combination of both methionine loading and determination of cystathionine synthase activity in cultured fibroblasts identified all of these carriers.

Pyridoxine treatment does not prevent homocystinemia after methionine loading in adult homocystinuria patients

Fasting homocystinemia in homocystinuria due to cystathionine synthase deficiency reportedly disappears on high-dose pyridoxine treatment. This does not necessarily reflect normal tolerance to methionine. The present study compares the effects of oral methionine loading on homocystine, cystine, and homocysteine-cysteine disulphide profiles in 8 adult homozygous homocystinuria patients on and off pyridoxine treatment and in 20 controls. Pyridoxine nearly normalized fasting serum amino acid levels. Nevertheless, with a similar methionine load the patient's homocystine levels on and off treatment rose and the cystine levels decreased, reflecting the ongoing formation of the homocysteine-cysteine disulphide in the presence of impaired transsulphuration of homocysteine. In the controls homocystinemia remained virtually absent and cystine transiently rose which indicates normal transsulphuration. On treatment methionine loading evoked a brisk rise of the homocysteine-cysteine disulphide levels to values equal to those off treatment, when these levels virtually plateaued after the load. Thus, pyridoxine treatment attenuates the biochemical abnormalities in the fasting patients but leaves their impaired capacity to handle major methionine loads essentially unchanged.

Nutritional aspect of methionine isomers studied by pulmonary exhalation of dimethyl sulfide and urinary excretion of alpha-keto-gamma-methiolbutyrate in humans.

With the view of evaluating the relative utilization of methionine isomers, the pulmonary exhalation of dimethyl sulfide and methyl mercaptan and the urinary excretion of alpha-keto-gamma-methiolbutyrate were studied in normal healthy subjects following oral or intravenous administration of L- or D-isomer of methionine. Dimethyl sulfide concentration in the expired alveolar gas (mean +/- SD) before methionine loading was 2.1 +/- 1.4 ng/dl (N = 23). Maximum concentrations of dimethyl sulfide in oral methionine loading tests were: 56.0 +/- 24.9 ng/dl (D-2g) (n = 4), 66.0 +/- 42.1 (D-1g) (N = 6) and 4.6 +/- 2.6 (L-2g) (N = 4); and in intravenous loading tests: 60.0 +/- 19.9 (D-Ig) (N = 4) and 4.2 +/- 2.1 (L-2g) (N = 3), respectively. The changes in methyl mercaptan in the expired alveolar gas were small and were disproportional to the changes in dimethyl sulfide following administration of both isomers. Preloading concentration of alpha-keto-gamma-methiolbutyrate in urine was 0.15 +/- 0.10 microgram/mg Creatinine (mean +/- SD) (N = 5). Postloading values during the initial two hours were 578 and 156 micrograms/mg Creatinine following 3g of D- and 0.20 and 29.7 micrograms/mg Creatinine following 3g of L-methionine ingestion. In view of the results obtained, significant amounts of D-methionine seem to be metabolized through the transaminative pathway of methionine metabolism.

Evidence for impairment of transsulfuration pathway in cirrhosis

Cirrhotic patients display fasting hypermethioninemia and delayed plasma clearance of methionine. These findings could be explained by an impairment in the main hepatic (“transsulfuration”) pathway for methionine degradation: methionine → S-adenosylmethionine → homocystine → cystathionine → cyst(e)ine → sulfate and taurine. In this study we quantified by pharmacokinetic methods the degree of methionine intolerance after an oral methionine load. The site of metabolic impairment was investigated by measuring the aforementioned amino acids in the venous blood plasma and urine, plus urinary sulfate excretion. In normal subjects, fasting plasma methionine concentration was 26.1 ± 2.5 μmol/L (mean ± SEM). After an oral load the methionine elimination half-life (t1/2) was 146 ± 10 min. Urine SO42− excretion averaged 632 ± 92 μmol/h before oral load; this rate increased to 1528 ± 248 μmol/h at 0–6 h and 1881 ± 214 μmol/h at 7–12 h after the load. Intermediates (i.e., homocystine, cystathionine, homoserine, or cystine) did not accumulate in the blood or urine after methionine administration. In cirrhotic patients basal plasma methionine concentration at 65.0 ± 19.4 μmole/L, and t1/2 at 458 ± 81 min were significantly elevated (p < 0.01) above normal. Urine SO42− excretion, initially 664 ± 84 μmol/h, increased after the load to 809 ± 64 μmol/h at 0–6 h and to 1201 ± 107 μmol/h at 7–12 h, but was significantly less than in normals in both periods (p < 0.05). Intermediary metabolites did not accumulate in blood or urine before or after the oral methionine load. Although the normals excreted 71% of the load of methionine sulfur as urinary SO42− within 24 h, cirrhotic patients excreted only 38%. The clearance of methionine after an oral load is retarded in cirrhosis. The simultaneous delay in SO42− excretion implies a block in the transsulfuration pathway. Because intermediates do not accumulate in plasma or urine during the prolonged hypermethioninemia after oral loading, the block appears to be above the level of homocystine synthesis.