Wood alcohol is toxic to humans but not to horses. Explain why, and also explain why wood alcohol poisoning in humans is treated by having the patient consume ethanol.

The ingestion of methanol, commonly known as wood alcohol, poses a serious threat to human health. Once absorbed into the system, the body's natural metabolic processes begin to transform methanol into dangerous compounds. Treatment for methanol poisoning centers around limiting this conversion. One medical intervention is the administration of intravenous or oral ethanol.

This approach operates on the basis of competitive inhibition, wherein ethanol competes with methanol for binding sites on the alcohol dehydrogenase enzyme. With ethanol present, it becomes the preferred substrate for the enzyme, allowing methanol to be excreted by the kidneys without being metabolized into its toxic byproducts, formaldehyde and formic acid.

Other treatments include using fomepizole, another competitive inhibitor, and supportive measures such as administering bicarbonate for acidosis. In severe cases, hemodialysis may be necessary to remove methanol and its metabolites more rapidly.

A key player in the metabolism of alcohol is the enzyme alcohol dehydrogenase (ADH). Located primarily in the liver, ADH belongs to a group of dehydrogenase enzymes that facilitate the breakdown of alcohols into aldehydes or ketones, subsequently making them easier for the body to eliminate.

In the case of methanol, ADH converts it into formaldehyde. However, differences in ADH between species can account for varying sensitivities to substances like wood alcohol. For example, horses have a form of ADH less efficient at converting methanol into its toxic byproducts, rendering them less susceptible to methanol toxicity compared to humans.

In biochemistry, competitive inhibition occurs when two different substances vie for the same binding site on an enzyme. Ethanol acts as a competitive inhibitor when introduced into a system that has also ingested methanol. Because ethanol and methanol are structurally similar and are both substrates for ADH, the intake of ethanol effectively hogs the active sites on the enzyme.

Given a high enough concentration, ethanol will be preferentially metabolized, delaying methanol catabolism and its subsequent toxicity. Higher affinity and the larger amount of ethanol compared to methanol ensure that ethanol is metabolized first, buying vital time to process and excrete methanol unmetabolized.

The metabolism of methanol in the human body is a two-step process facilitated by the liver enzymes. Initially, methanol is metabolized by alcohol dehydrogenase into formaldehyde, which is then further metabolized to formic acid by the enzyme formaldehyde dehydrogenase.

Both metabolites of methanol—formaldehyde and formic acid—are toxic, with the latter causing metabolic acidosis and potential damage to the central nervous system and optic nerve. The process of methanol metabolism and its detrimental outcomes emphasize the importance of prompt treatment in order to avoid serious health consequences.