Average people produce an enzyme called phenylalanine hydroxylase in their bodies. When they ingest phenylalanine, the body cannot use it, so this enzyme changes phenylalanine into tyrosine, which the body can use. In people with PKU, very little or no enzyme is made. Since the phenylalanine they eat is not being changed into tyrosine, it builds up in their bodies. This can cause problems with the way their bodies work.