Effects of
tumor, operative stress and
tumor removal, and postoperative TPN of varying
amino acid compositions on brain levels of
tryptophan or
tyrosine as predicted by their brain influx rates were studied in normals and in malnourished
cancer patients. Concentrations of the large
neutral amino acids (LNAA) were determined in patients before and after
tumor removal, and in postoperative patients before and after receiving either a standard TPN
solution (STD-TPN), or a
branched-chain amino acid solution (BCAA-TPN). The LNAA were altered in all groups versus normals. Brain influx rates showed the following: in preoperative patients, predicted brain
tryptophan levels were below normal (P less than 0.001), whereas
tyrosine levels were within or above normal; no significant differences between pre- and postoperative
tryptophan or
tyrosine levels; postoperative STD-TPN did not change predicted brain
tryptophan concentration from preinfusion values, but BCAA-TPN decreased it (P less than 0.001), underscoring the common transport carrier; and preinfusion predicted brain
tyrosine levels were decreased (P less than 0.001) by both types of TPN solutions. These results imply low substrate levels for brain
serotonin and
catecholamine synthesis, possibly affecting functions dependent on their control.