On postnatal day @ , we performed a stable isotope study with -LSB- @ - ( @ ) C -RSB- phenylalanine , -LSB- ring-D@ -RSB- tyrosine , -LSB- U - ( @ ) C@ , ( @ ) N -RSB- leucine , and -LSB- methyl-D@ -RSB- - ketoisocaproic acid to quantify intermediate amino acid metabolism .