Inborn errors of metabolism:

Two inborn errors of metabolism are known that affect phenylalanine metabolism. This is inherited metabolic disorders caused through the absence of one of the enzymes in the pathway. One of them disorders, alkaptonuria, is caused through the absence of homogentisate oxidase. This concludes in the accumulation of homogentisate that is subsequently excreted in the urine and that oxidizes to a black color on standing.  That defect is harmless, in contrast with the other disorder and phenylketonuria. In phenylketonuria there is a block in the hydroxylation of phenylalanine to tyrosine caused by an absence or deficiency of phenylalanine hydroxylase or, more rarely and of its tetrahydrobiopterin coenzyme.  The conclusion is a 20-fold raise in the stages of phenylalanine in the blood with its subsequent transamination to phenylpyruvate.  If untreated, severe mental retardation happens, with a life expectancy of on average 20 years. Within an incidence  of  1:20 000  this  condition  is  now  screened  for  at  birth and  with  treatment being to restrict the intake of phenylalanine  (by putting the individual on a low phenylalanine   diet)  and  therefore  minimize  the  required  to  metabolize   the  excess. Moreover, enough phenylalanine must be gives to meet the required for replacement and growth.