In fish skeletal muscle, Cathepsin D is the major intracellular lysosomal endopeptidase belonging to the Aspartic acid protease family (active at acidic pH ~3.0–4.0). Cathepsins B, H, and L are cysteine/thiol proteases, and Cathepsin A is a serine carboxypeptidase.

Assertion (A) is true: The epidermal mucus of harvested fish contains mucins, free amino acids, soluble proteins (albumins), lipoids, and phosphatides. Reason (R) is false: Far from inhibiting microbes, this nutrient-dense aqueous surface layer provides an ideal substrate and culture medium for psychrotrophic spoilage bacteria (Pseudomonas, Shewanella), which rapidly proliferate and penetrate muscle tissue.

Statement (I) is correct: Post-mortem accumulation of lactic acid drives muscle pH downward toward the isoelectric point of actomyosin (~5.3–5.5), neutralizing surface net electrical charge and drastically reducing muscle protein water holding capacity (WHC). Statement (II) is completely incorrect: pH is one of the most critical thermodynamic determinants governing protein quaternary structure, net charge, denaturation, and enzymatic activity.