In hamster heart, the majority of the
phosphatidylcholine is synthesized via the
CDP-choline pathway, and the rate-limiting step of this pathway is catalysed by
CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15). We have shown previously [Choy (1982) J. Biol. Chem. 257, 10928-10933] that, in the myopathic heart, the level of cardiac
CTP was diminished during the development of the disease. In order to maintain the level of
CDP-choline, and consequently the rate of
phosphatidylcholine biosynthesis, cardiac cytidylyltransferase activity was increased. However, it was not clear if the same compensatory mechanism would occur when the cardiac
CTP level was decreased rapidly. In this study,
hypoxia of the hamster heart was produced by perfusion with
buffer saturated with 95% N2. The heart was pulse-labelled with radioactive
choline and then chased with non-radioactive
choline for various periods under hypoxic conditions. There was a severe decrease in
ATP and
CTP levels within 60 min of hypoxic perfusion, with a corresponding fall in the rate of
phosphatidylcholine biosynthesis. Analysis of the
choline-containing metabolites revealed that the lowered
ATP level did not affect the phosphorylation of
choline to
phosphocholine, but the lower
CTP level resulted in the decreased conversion of
phosphocholine to
CDP-choline. Determination of
enzyme activities revealed that hypoxic treatment resulted in the enhanced translocation of cytidylyltransferase from the cytosolic to the microsomal form. This enhanced translocation was probably caused by the accumulation of
fatty acids in the heart during
hypoxia. We postulate that the enhancement of translocation of the cytidylyltransferase to the microsomal form (a more active form) is a mechanism by which the heart can compensate for the decrease in
CTP level during
hypoxia in order to maintain
phosphatidylcholine biosynthesis.