Tissue Distribution of β-Cyanoalanine Synthase in Leaves

Abstract

ABSTRACI,j-Cyanoalanine synthase, which catalyzes the reaction between cysteine and HCN to form 6-cyanoalanine and H2S, was assayed in leaf tissues from cyanogenic (Sorghum bicolor x Sorgkum sudawease [sorghum]) and noncyanogenic (Pisum sativum Ipeal Zea mays [maizel, and Allium porrum Pleek]) plants. The activity in whole kaf extracts ranged from 33 nanomoles per gram fresh weight per minute in leeks, to 1940 nanomoles per gram fresh weight per minute in sorghum. The specific activities of 6-cyanoalanine synthase in epidermal protoplasts from maize and sorghum and in epidermal tissues from peas were in each case greater than the corresponding values for mesophyll protoplasts or tissues, or for strands of bundle sheath cells.The tissue distributions for this enzyme were determined for pea, leek, and sorghum: the mesophyll protoplasts and tissues in these three plants contained 65% to 78% of the enzyme, while epidermal protoplasts and tissues contained 10% to 35% of the total leaf activity. In sorghum, the bundle sheath strands contained 13% of the leaf activity. The presence of,-cyanoalanine synthase in all tissues and species studied suggests a fundamental role for this enzyme in plant metabolism.13-Cyanoalanine synthase (EC 4.4.1.9), which catalyzes the reaction between cysteine and HCN to form (l-cyanoalanine and H2S (4), is widely distributed in higher plants (4,11,16). Although the enzyme has been purified and its kinetic properties examined (1, 12), its role(s) in plant metabolism have not yet been defined (see 8 for review). ,3-Cyanoalanine hydratase (EC 4.2.1.65) catalyzes the hydration of (B-cyanoalanine to form asparagine (6). The sequential action of these two enzymes constitutes an effective mechanism for detoxifying HCN while conserving the nitrogen atom in the form of amide nitrogen. Such a role for these enzymes in cyanide detoxification is supported by studies (4,5,17) showing that gaseous, '4C-labeled HCN is incorporated in high yields into asparagine when administered to either cyanogenic or noncyanogenic species. In addition, experiments demonstrating the detoxification of HCN produced in vivo during the metabolic turnover of cyanogenic glycosides in Lotus sp., Nandina domestica, and Manihot utilissima have been described (2, 3,17). Asparagine synthesis in Asparagus officinalis has also been attributed to this two-step metabolic sequence (9), but the source of the HCN required as a substrate was not identified.Recent studies by Peiser et aL (19)

Cyanide Metabolism Linked with Ethylene Biosynthesis in Ripening Apple Fruit

“…When exogenously applied to plant tissues, HCN is effectively converted to /3-cyanoalanine (6,20,33,34), which is further hydrolyzed to asparagine(9).…”

“…~3-Cyanoalanine synthase, which catalyzes the reaction between cysteine and HCN to from j3-cyanoalanine and H2S, is widely distributed not only in higher plants(6, 13, 14. L 17, 20,33,34), but also in algae(14) and microorganisms(7, 8,10,12).…”

Enzymes of Glucose Oxidation in Leaf Tissues

“…The Chl:protein ratio in the mesophyll cells from both plants was similar; however, the sorghum bundle sheath strands showed a lower Chl:protein ratio than the strands from maize. The distribution of Chl and protein among the leaf tissues of sorghum was calculated (29). The mesophyll protoplasts contained 75% of the Chl and nearly 66% ofthe total protein ofthe leaf.…”

“…The separated tissues were weighed and immediately frozen in liquid N2. All tissues were homogenized in Tris buffer (23), and low mol wt molecules were removed by passage through Sephadex G-25 which was equilibrated with the identical buffer (29). Recovery of proteins was >90%.…”

Structure of Soybean β-Cyanoalanine Synthase and the Molecular Basis for Cyanide Detoxification in Plants

“…The activity of this enzyme is widespread in plant tissues (Wurtele et al, 1984(Wurtele et al, , 1985 and localized to mitochondria (Wurtele et al, 1985;Hatzfeld et al, 2000;Maruyama et al, 2000;Hawkesford, 2000, 2002;Lai et al, 2009). Thus, CAS activity is induced and localized to effectively protect the electron transport chain at the site of ethylene production (Bleecker and Kende, 2000).…”