An interesting and important example of competitive substrate inhibition is the enzyme alcohol dehydrogenase (ALD) in the presence of ethanol and methanol. If a person ingests methanol, ALD will convert it to form formaldehyde and then formate which causes blindness. Consequently, the treatment involves intravenously injecting ethanol (which is metabolized at a slower rate than methanol) at a controlled rate to tie up ALD to slow the metabolism of methanol-to-formaldehyde-to-formulate so that the kidneys have time to filter out the methanol which is then excreted in the urine. With this treatment blindness is avoided. For more on the methanol/ethanol competitive inhibition see Problem P7-21.