The activity of phosphoenolpyruvate carboxykinase throughout the lactation cycle of the guinea pig mammary gland

The enzyme phosphoenolpyruvate carboxykinase (PEPCK) has been measured in the guinea pig mammary gland throughout the pregnancy-lactation cycle. This is of interest since the primary importance of PEPCK is thought to be its role in gluconeogenesis and it is questionable whether or not gluconeogenesis occurs in the mammary gland. The enzyme activity, present in both the cytosol and mitochondria, was shown to follow the lactation profile. During the transition into lactation, cytosolic PEPCK activity increases 11-fold and mitochondrial PEPCK activity 43-fold while tissue weight increases 4-fold. Fructose 1,6-bisphosphatase was found to increase at a rate only slightly greater than that of the tissue weight. The increase in mitochondrial PEPCK activity is thus about 10 times greater than that of general tissue expansion, whereas the cytosolic PEPCK activity increase is only 2-fold greater. The activity of fructose 1,6-bisphosphatase appears to be merely keeping pace with general tissue expansion. The mitochondrial enzyme constitutes 59 +/- 3% of the total gland PEPCK activity in the prepartum state and 86 +/- 2% at midlactation. Therefore, mitochondrial PEPCK is the isoenzyme undergoing the greater increase during the transition into lactation in the guinea pig mammary gland and thus would appear to play the more important role in the conversion of oxalacetate to phosphoenolpyruvate in this tissue.