Metabolism of xenobiotics using amino acids or glutathione

5 Metabolism of xenobiotics using amino acids or glutathione 

Uric acid, ammonia and to a lesser extent urea are the principal forms in which surplus nitrogen is excreted in birds. In addition to these substances, nitrogen from certain amino acids and glutathione are con- 

Five different subunits (CL1-CL5) of glutathione-S-transferase, each 27 kDa, have been separated from chick liver cytosol (Chang ei al, 1990). Of these, CL2, CL3 and CL5 can form homodimers, whereas CLl and CL4 exist only as the heterodimers CL 1-2 and CL3—4. One of these has been purified and has a Km for glutathione of 0.35 mM. Since the likely intracellular glutathione concentration is 2.5—5.0 mM, it suggests that glutathione will not be rate limiting in 

The enzyme rhodanese (thiosulphate sulphurtran-sferase, EC.2.8.1.1) has a wide distribution among vertebrate tissues, and catalyses the conversion of cyanide into the less toxic thiocyanate  

Birds and reptiles are oviparous, and the cleidoic eggs that they produce contain all the nutrient required until hatching. This nutrient, which is mainly protein and lipoprotein, is synthesised in the liver and oviduct prior to oviposition. Lipoproteins are discussed in Section 4.5, and the control of egg protein synthesis in Section 10.3. Birds excrete a semi-solid urine, and this requires a lower water intake than is possible in ureotelic animals. The metabolic adaption that allows this to occur is the excretion of nitrogen principally in the form of uric acid. Uric acid is sparingly soluble in water and is present in avian ureters largely as a colloidal suspension. This is discussed in Section 5.4. 

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