5.13: Phase II Reactions: Miscellaneous Conjugation Reactions

Phase II biotransformations are detoxification mechanisms that conjugate xenobiotics with endogenous substances, neutralizing their toxicity.

A key example involves the conjugation of cyanide ions, which impair cellular respiration and alter hemoglobin into non-oxygen-carrying cyanmethemoglobin. To neutralize this threat, a sulfur atom from thiosulphate is transferred to the cyanide ion, catalyzed by the enzyme rhodanese, resulting in an inactive compound called thiocyanate. The production of thiosulfate occurs through the transamination of β-cysteine to mercaptopyruvate.

Another significant phase II reaction involves conjugating endogenous purine and pyrimidine bases with ribose to create nucleotides. These nucleotides play a crucial role in protein synthesis and energy metabolism, highlighting the importance of this biotransformation process.

Finally, taurine, a β-amino sulphonic acid, contributes significantly to bile production. It conjugates with endogenous bile acids, adding to the composition of bile, which plays a pivotal role in digestion.

Phase II conjugation reactions neutralize harmful substances, support essential biological processes, and contribute to digestive functions, demonstrating their importance in maintaining overall health.

Phase II biotransformations primarily involve detoxification processes where xenobiotics conjugate with endogenous substances.

The cyanide ion's toxicity stems from its capacity to inhibit vital cellular respiration enzymes and transform hemoglobin into cyanomethemoglobin, which cannot deliver oxygen to tissues.

The body neutralizes cyanide ions via conjugation. In this process, a sulfur atom from thiosulphate is transferred to the cyanide ion in the presence of the enzyme rhodanese, forming an inactive thiocyanate.

Thiosulfate is produced via a biochemical process where β-cysteine is transaminated to mercaptopyruvate.

Another phase II conjugation involves endogenous purine and pyrimidine bases conjugating with ribose, forming nucleotides. This process is vital for essential biological functions like protein synthesis and energy metabolism.

Lastly, taurine, a β-amino sulphonic acid, plays a pivotal role in bile production. It conjugates with endogenous bile acids, contributing to the composition of bile, a crucial element in digestion.