The Glucose-Alanine cycle (Cahill cycle) operates between muscle and liver. In skeletal muscle, toxic ammonia produced from amino acid catabolism is transferred to pyruvate to form non-toxic alanine, which travels through blood to the liver where it is deaminated back to pyruvate for gluconeogenesis while ammonia enters the urea cycle.

Corticotropin (ACTH), Dopamine (via D1-like GPCRs), and Histamine (via H2 receptors) activate adenylyl cyclase to generate cyclic AMP (cAMP) as a second messenger. In contrast, Progesterone is a lipid-soluble steroid hormone that diffuses into cells and binds nuclear receptors, without using cell-surface GPCR-cAMP signaling.

Inhibition of mitochondrial electron transport chain by Piericidin A:

• Assertion (A) is true: Piericidin A is a microbial antibiotic structurally homologous to ubiquinone (coenzyme Q) that potently arrests mitochondrial respiration and ATP synthesis.
• Reason (R) is true: Piericidin A competitively binds to the ubiquinone reduction site in NADH:ubiquinone oxidoreductase (Complex I), blocking electron transfer from iron-sulfur clusters (N2) to ubiquinone.
• Because blocking Complex I electron entry into the ubiquinone pool arrests the downstream cascade of the respiratory chain when NADH is the primary electron donor, Reason (R) directly explains Assertion (A).

Terminal step of the hepatic urea (ornithine) cycle:

• Arginase (specifically cytosolic arginase I) catalyzes the hydrolytic cleavage of the guanidino group of L-arginine to produce urea and regenerate L-ornithine, which re-enters the mitochondrial matrix for subsequent cycles.
• Argininosuccinase (argininosuccinate lyase) cleaves argininosuccinate into arginine and fumarate.
• Argininosuccinate synthetase condenses citrulline and aspartate with ATP consumption to yield argininosuccinate.
• Ornithine transcarbamoylase (OTC) transfers a carbamoyl group from carbamoyl phosphate onto ornithine to form citrulline.