The ratio of plasma tyrosine plus phenylalanine ( tyrosine precursor ) to amino acids competing for brain uptake ( free-tryptophan , leucine , isoleucine , valine , methionine , threonine , and lysine ) increased @-fold from rest in TYR and remained elevated throughout exercise ( P < @ ) , whereas it declined in PLA from rest to preexercise ( P = @ ) .