Final answer:
CYP2D6 is the enzyme that is largely responsible for the conversion of codeine to morphine in the body, converting the less potent opioid into its active form.
Step-by-step explanation:
The enzyme largely responsible for the conversion of codeine to morphine in the body is CYP2D6. Codeine undergoes metabolic transformation to various metabolites through the action of liver enzymes. One critical metabolic pathway involves the cytochrome P450 enzyme system, particularly the isoform CYP2D6, which effectively demethylates codeine to its active form, morphine. Morphine is a potent opioid receptor agonist providing analgesic effects. Genetic polymorphisms in the CYP2D6 gene can lead to variability in patients' response to codeine due to differences in the efficacy of the metabolic pathway.
Morphine and codeine, both obtained from the opium poppy plant, are used for pain relief, while heroin is a derivative of morphine with addictive properties. Medications like OxyContin, which are structurally similar to these opiates, also share similar physiological effects due to their action on the same opioid receptors. The correct answer to the question about the enzyme which converts codeine to morphine is A) CYP2D6.