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J Biol Chem, Vol. 273, Issue 52, 34760-34769, December 25, 1998
From the Department of Biochemistry and Molecular Biology, The
Pennsylvania State University College of Medicine,
Hershey, Pennsylvania 17033
Tyrosine in an hepatocyte is transported from the
plasma, synthesized from phenylalanine, or released during protein
turnover. Effects of phenylalanine and tyrosine on the formation and
fate (partitioning) of tyrosine from the different sources were
examined in primary rat hepatocyte cultures. Rates of tyrosine
degradation, transport, incorporation into and release from protein,
and synthesis from phenylalanine were measured as well as the
intracellular dilution of labeled tyrosine and phenylalanine
incorporated into protein. We found tyrosine had little effect on
phenylalanine hydroxylation over a wide range of conditions, that
transported tyrosine and tyrosine from phenylalanine are in different
metabolic pools, and that there appears to be channeling of newly
synthesized tyrosine during degradation. In addition, under some
conditions, intracellular partitioning of tyrosine is determined by
tyrosine concentration. Specifically, if extracellular tyrosine is low and phenylalanine is at a normal plasma level, tyrosine use in protein
synthesis takes precedence over tyrosine degradation or export. It is
proposed that the mechanism controlling this is kinetic, based on
relative rates of tyrosyl-tRNA formation and tyrosine degradation and
export. A quantitative model of tyrosine and phenylalanine in-flow and
out-flow in hepatocytes is given, incorporating tyrosine synthesis,
degradation, plasma membrane transport, and tyrosine and phenylalanine
use and release during protein turnover.
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