Relative roles of albumin and ceruloplasmin in the formation of homocystine, homocysteine-cysteine-mixed disulfide, and cystine in circulation.

Disulfide forms of homocysteine account for >98% of total homocysteine in plasma from healthy individuals. We recently reported that homocysteine reacts with albumin-Cys(34)-S-S-cysteine to form homocysteine-cysteine mixed disulfide and albumin-Cys(34) thiolate anion. The latter then reacts with homocystine or homocysteine-cysteine mixed disulfide to form albumin-bound homocysteine (Sengupta, S., Chen, H., Togawa, T., DiBello, P. M., Majors, A. K., Büdy, B., Ketterer, M. E., and Jacobsen, D. W. (2001) J. Biol. Chem. 276, 30111-30117). We now extend these studies to show that human albumin, but not ceruloplasmin, mediates the conversion of homocysteine to its low molecular weight disulfide forms (homocystine and homocysteine-cysteine mixed disulfide) by thiol/disulfide exchange reactions. Only a small fraction of homocystine is formed by an oxidative process in which copper bound to albumin, but not ceruloplasmin, mediates the reaction. When copper is removed from albumin by chelation, the overall conversion of homocysteine to its disulfide forms is reduced by only 20%. Ceruloplasmin was an ineffective catalyst of homocysteine oxidation, and immunoprecipitation of ceruloplasmin from human plasma did not inhibit the capacity of plasma to mediate the conversion of homocysteine to its disulfide forms. In contrast, ceruloplasmin was a highly efficient catalyst for the oxidation of cysteine and cysteinylglycine to cystine and bis(-S-cysteinylglycine), respectively. However, when thiols (cysteine and homocysteine) that are disulfide-bonded to albumin-Cys(34) are removed by treatment with dithiothreitol to form albumin-Cys(34)-SH (mercaptalbumin), the conversion of homocysteine to its disulfide forms is completely blocked. In conclusion, albumin mediates the formation of disulfide forms of homocysteine by thiol/disulfide exchange, whereas ceruloplasmin converts cysteine to cystine by copper-dependent autooxidation.

The specific features of methionine biosynthesis and metabolism in plants.

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

[Utilization of inorganic sulfur sources by Staphylococcus aureus strains]. / Verwertung anorganischer Schwefelquellen durch Staphylococcus aureus-Stämme.

Staphylococcus aureus strains of different host-adapted variants (Meyer 1966) have been tested for their ability to use inorganic sulfur sources. All the 25 strains tested were able to utilize sodium sulfide as sulfur source in a medium similar to that described by Kloos and Pattee (1965). Using S. aureus strain 116/74 grown in a medium containing Na2-35S as the only sulfur source we studied incorporation and insertion of inorganic sulfide into sulfur containing amino acids. In disintegrated and fractionated cellular material we could find 35S labelled homocystine and methionine as major compounds, and cystine, cysteic acid, homocysteic acid, and beta-sulphopyruvate as minor compounds. The occurrence of homocystine and the sulfonic acids in bacterial proteins is rather uncommon.