Inhibition of ubiquitin-protein ligase (E3) by mono- and bifunctional phenylarsenoxides. Evidence for essential vicinal thiols and a proximal nucleophile.

Trivalent arsenoxides bind to vicinal thiol groups of proteins. We showed previously that the simplest trivalent arsenoxide, inorganic arsenite, inhibits ubiquitin-dependent protein degradation in rabbit reticulocyte lysate (Klemperer, N.S., and Pickart, C.M. (1989) J. Biol. Chem. 264, 19245-19242). We now show that, relative to arsenite, phenylarsenoxides are 10-165-fold more potent inhibitors of protein degradation in the same system (K0.5 for inhibition by p-aminophenylarsenoxide was 3.5-20 microM, depending on the substrate). In the ubiquitin-dependent proteolytic pathway, covalent ligation of ubiquitin to protein substrates targets the latter for degradation. In certain cases, specificity in ubiquitin-substrate conjugation depends critically upon the properties of ubiquitin-protein ligase or E3. Among other effects, p-aminophenylarsenoxide decreased the steady-state level of ubiquitinated human alpha-lactalbumin; this is a substrate which is acted upon directly by ubiquitin-protein ligase-alpha (E3-alpha). This finding suggests that phenylarsenoxides (unlike arsenite) inhibit E3. Several other lines of evidence confirm this conclusion. 1) A complex of E3-alpha and the 14-kDa ubiquitin-conjugating (E2) isozyme binds to phenylarsenoxide-Sepharose resin, with the E3 component of the complex mediating binding. 2) p-Aminophenylarsenoxide inhibited isolated E3 (K0.5 approximately 50 microM); inhibition was readily reversed by addition of dithiothreitol (which contains a competing vicinal thiol group), but not by beta-mercaptoethylamine (a monothiol). 3) A bifunctional phenylarsenoxide (bromoacetylaminophenylarsenoxide) rapidly and irreversibly inactivated E3; bromoacetyl aniline, which lacks an arsenoxide moiety, did not inhibit E3. These results suggest that E3 possesses essential vicinal thiol groups and that there is a reactive nucleophile proximal to the vicinal thiol site. The bifunctional phenylarsenoxide should be a useful tool for probing the relationship between structure and function in E3. As expected from prior results with arsenite, p-aminophenylarsenoxide was also a potent inhibitor of the turnover of ubiquitin-(human) alpha-lactalbumin conjugates.     

Impact of Atmospheric Sulfur Deposition on Sulfur Metabolism in Plants: H2S as Sulfur Source for Sulfur Deprived Brassica oleracea L.

Brassica oleracea L. was rather insensitive to atmospheric H₂S: growth was only negatively affected at ≥0.4 μl I^?1. Shoots formed a sink for H₂S and the uptake rate showed saturation kinetics with respect to the atmospheric concentration. The H₂S uptake rate was high in comparison with other species, which may reflect the high sulfur need of Brassica. The net uptake of sulfate by roots of hydroponically grown plants was substantially reduced after one week of exposure to 0.25 μl l^?1 H₂S, indicating that plants switched in part from sulfate to H₂S as sulfur source for plant growth. Plants were sulfur deficient after two weeks of sulfur deprivation, illustrated by reduced growth, which was more pronounced for shoots than for roots, and in enhanced shoot dry matter content. The latter could for the greater part be attributed to enhanced levels of soluble sugars and starch. Sulfur deficiency was further characterized by a low pigment content, extremely low levels of sulfate and water-soluble non-protein thiols, and by enhanced levels of nitrate and free amino acids, particularly in the shoots. Furthermore, sulfur deficient plants contained a lower total lipid content in shoots, whereas its content in roots was unaffected. The level of sulfolipids was decreased in both roots and shoots. When sulfur deprived plants were exposed to 0.25 μl I^?1 H₂S for one week, all sulfur deficiency symptoms were abolished and growth was restored. Furthermore, plants were able to grow with 0.4 μl I^?1 H₂S as the sole sulfur source. Water-soluble non-protein thiol content was enhanced in both shoots and roots of H₂S exposed plants, irrespective of the sulfate supply to the roots, whereas plants grown with H₂S as sole sulfur source contained very low sulfate levels. The interaction between atmospheric and pedospheric sulfur nutrition in plants is discussed.