Cyanidioschyzon enzymes need not be regulated as stringently as f

Cyanidioschyzon enzymes need not be regulated as MDV3100 purchase stringently as for Chlamydomonas and Synechococcus given that sulfur would never normally become limiting in its native environment where it could utilize the sulfur assimilation pathway for metal detoxification

without experiencing the threat of sulfur depletion. Lending support to this notion is that this red alga possesses one additional SAT and two additional OASTL homologues [55]. However, it synthesized more CdS only under sulfate-, PP2 concentration and not sulfite- or cysteine-supplemented conditions in a similar manner to Chlamydomonas, and in contrast to Synechococcus where all conditions gave significant increases in acid labile sulfide production (Figure 2C). The extracted activity of SAT-OASTL indicates that these enzymes do play a role in the production of required assimilated sulfur for cadmium sulfide as it was highest when cells were exposed to Cd(II) without sulfate supplementation. Higher plants that have been genetically engineered to have higher levels of these enzymes have shown some increased resistance to Cd(II) [11, 56] and other metals [57]. Bearing in mind that in vivo activity would be distinct from extracted activity because

of, among other things, different substrate concentrations, it is likely that sulfur flux through SAT-OASTL would be higher in the sulfate supplemented cells, which could contribute to the respective elevated CdS production. Enzymatic sulfide production

Hydrogen sulfide, traditionally considered a toxic compound, has recently been implicated in cellular signaling [58, 59]. However, it would be expected that metal sulfide biosynthesis should require more sulfide than signaling processes. Several metabolic sources of sulfide have been proposed [60] and of these, cysteine desulfhydrase activity is the most evident and is accentuated by feeding with cysteine [61]. In addition, there is some evidence that sulfide is released during excess sulfate nutrition which can be through provision of sulfate or sulfur dioxide/sulfite [62]. This appears to be because of inadequate cellular supplies of O-acetylserine [63] and as such, Vasopressin Receptor OASTL cannot utilize all the H2S generated by sulfite reductase. This could have occurred in the sulfate supplemented cultures of this study, particularly in the case of Cyanidioschyzon where the sulfate concentration was 108.6 mM, however sulfate in the media of the other two species was relatively low. Other metabolic sources of significant amounts of H2S are speculative. The assayed activity of cysteine desulfhydrase was generally much higher in Cyanidioschyzon than in Chlamydomonas and Synechococcus (Figure 4) as was the case for SAT-OASTL (Figure 3). This further indicates adaptation to high sulfur environments that accounts for its elevated ability to biotransform Cd(II) into metal sulfide which is insoluble and therefore, non-toxic.

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