Sulfur- and seleno-containing amino acids.

This special issue is devoted to professor Toshihiko Ubuka (1934–2008), a distinguished Japanese biochemist who, in his scientific life, researched the metabolism and function of cysteine, sulfur nutrition, modification and renaturation of proteins and the possible significance of cysteine metabolites in antioxidation processes in liver cell mitochondria. Recollections about professor Ubuka, written by M. Wróbel and professor Ubuka’s wife and son (Wróbel et al. 2011), open this volume.

Sulfur is one of the prevalent elements in the human body and is present, most often, in the sulfur-containing amino acids: methionine, cysteine, homocysteine (and related disulfides, cystine and homocystine), and taurine. Selenium, in the form of selenocysteine, occurs in 25 proteins in the human proteome (Kryukov et al. 2003).

This volume contains a selection of contributions originally presented at the 11th International Congress on Amino Acids and Proteins held in Vienna, August 3–7, 2009. However, additional manuscripts dedicated to the sulfur- and seleno-containing compounds submitted subsequent to the Congress have also been included. Topics presented in reviews illustrate the present challenges in this area of research and should stimulate further investigations. The reviews begin with a very important opening review about pyridoxal 5′-phosphate-dependent enzymes currently known to catalyze cysteine and selenocysteine conjugates β-lyase reactions, presented by Cooper et al. (2011), one of the fathers of this field, and by Pinto et al. (2011). Four reviews, presented by J. Papenbrock, N. Nagahara, R. Hondal, and K.A. Ahmed, concern the role of sulfur and selenium in enzymatic proteins: sulfur transferases (Papenbrock et al. 2011; Nagahara 2011) and thioredoxin reductase (Hondal and Ruggles 2011) and redox-sensitive proteins (Ahmed et al. 2011). Thiol dioxygenases, proteins in the cupin superfamily, were described by Stipanuk et al. (2011), the expert in the field of cysteine dioxygenase. The benefits of sulfur compounds used in health products (popular items include the active ingredients of garlic) or hydrogen sulfide in cellular signaling are presented by Melino et al. (2011) and Kimura (2011), respectively.

The second part of this issue is a group of original articles that investigated the sulfur signaling agent (HS− or S°), its generation and its possible role in the regulation of cell proliferation (Jurkowska et al. 2011a, b; Cartini et al. 2011); how cells respond to cysteine deprivation (Sikalidis et al. 2011); the role of cysteine residues in the redox-regulated porphobilinogen synthase activation (Sawada et al. 2011); and the formation and determination of some derivatives of sulfur-containing amino acids (Choudhary et al. 2011; Xu and Xu 2011; Glowacki et al. 2011).

We hope that the articles published in this special issue will continue to generate an interest in such unique biological agents as sulfane sulfur and hydrogen sulfide, shed light on a new aspect of regulation of protein function via the sulfur amino acids, and will further our understanding of the sulfur amino acids.