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.