[84] Preparation and determination of sulfur amino acids and related compounds

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Iron-dependent formation of homocysteine from methionine and other thioethers

OBJECTIVE

We tested whether homocysteine is formed from methionine and other thioethers in vitro and in vivo, because methionine can be chemically demethylated to homocysteine.

DESIGN

In in vitro studies, chemical conversions of thioethers (methionine, S-adenosylhomocysteine and cystathionine) into homocysteine were measured under various aerobic conditions. In humans, oral methionine (0.17 mmol/kg body weight) loading tests with and without an oral iron dose (ferrous sulfate, 13 mumol/kg) were performed.

SETTING

A university setting in Birmingham, AL, USA.

SUBJECTS

A total of five healthy adult subjects volunteered.

RESULTS

The in vitro incubation of methionine, S-adenosylhomocysteine or cystathionine with chelated iron resulted in the formation of homocysteine. These conversions were iron- and pH-dependent (pH optima between 5.0 and 6.0) and it was also chelator-dependent. In humans, oral methionine loading tests resulted in a 45% increase in the area-under-the-curve for plasma total homocysteine concentrations, when iron was given together with methionine.

CONCLUSION

Our data suggest that iron-dependent chemical formation of homocysteine can occur in vivo, and contribute to the plasma total homocysteine pool, since this formation can occur ceaselessly. We hypothesize that plasma total homocysteine concentrations reflect, in part, non-protein-bound iron in the body.

Cystathionine beta-synthase and gamma-cystathionase in helminths

The activities of gamma-cystathionase and cystathionine beta-synthase were investigated in a range of gastrointestinal, free-living and entomophagous nematodes. Although nematode gamma-cystathionase used the same range of substrates as the mammalian hepatic enzyme, its activity was extremely low and there were significant interspecies variations with respect to the relative order of active substrates. Like the mammalian liver enzyme, nematode cystathionine beta-synthase showed activity in the directions of both cystathionine synthesis and the forward and reverse "L-serine sulphhydrase" reactions. However, the most important feature of the survey was the widespread ability of nematode cystathionine beta-synthase to catalyse the non-mammalian "activated L-serine sulphhydrase" reaction (L-cysteine + R-SH----cysteine thioether + H2S). Additional survey work revealed that the ability to catalyse the activated L-serine sulphhydrase reaction was almost universal amongst nematodes. Activated L-serine sulphhydrase activity was also demonstrated in the acanthocephalan Pomphorhynchus laevis but was absent from cestodes and digeneans.

Cysteine metabolism in the cestode Hymenolepis diminuta

The major pathways for cysteine catabolism in Hymenolepis diminuta have been investigated. The parasite has an active cystathionine-beta-synthase and, as in other tissues, this enzyme has a wide substrate specificity. However, the enzyme from H. diminuta differs significantly from the mammalian enzyme in showing a high serine sulphydrase activity and a high serine lyase activity. There was only low gamma-cystathionase activity in H. diminuta and again the enzyme showed a range of substrate specificities. Cysteine aminotransferase activity was readily demonstrated in the tapeworm, but there was no evidence for 3-mercaptopyruvate sulphotransferase activity. An oxidative pathway for cysteine catabolism in H. diminuta was shown by the presence of cysteine dioxygenase and cysteine sulphinate transaminase. The properties of the helminth cysteine dioxygenase were very similar to those of rat liver. H. diminuta was able to reduce cystine to cysteine via a glutathione-cysteine transhydrogenase system.