A proliferation-competent adult rat liver cell monolayer system has been analyzed for tissue-specific functions during its growth cycle. High levels of the adult (L type) form of pyruvate kinase (EC 2.7.1.40) and glutathione S-transferase B ("ligandin," EC 2.5.1.18) are observed during the early lag phase; they decline markedly during the logarithmic phase and reappear during the stationary phase. By contrast, elevated levels ofthe fetal (K type) form of pyruvate kinase and ai-fetoprotein production appear only after proliferation begins; this pattern diminishes slightly during stationary phase as the adult phenotype is restored. Albumin production continues throughout the entire growth cycle. These in vitro findings simulate those observed during hepatoproliferative transitions in the intact animal and, as such, constitute a developmental program for normal epithelial cells in primary culture. Many specialized animal cell functions are related to proliferative and developmental states (for review, see ref. 1). Much of this evidence comes from physiological studies of the liver (2-5). For example, the L-isozymic form of pyruvate kinase (EC 2.7.1.40) (6) and glutathione S-transferase B ("ligandin," EC 2.5.1.18) (7) are prominent hepatocyte properties associated with quiescent or adult states whereas the production of alfetoprotein (8) and the K-isozymic form of pyruvate kinase (6) are properties associated with proliferating or fetal states. Temporal aspects of the appearance or disappearance of differentiated properties during transitional states imply that distinct regulatory "programs" exist. However, the molecular and cellular bases of such programs are poorly understood and difficult to study in the intact animal. Hepatocyte tissue cultures offer a more direct approach (9) (15,16). Additional functional and proliferative properties of this system have been reported (9,15,17).Pyruvate Kinase Isozymes. At indicated times, culture media were removed and monolayers (10-20 dishes per point) were washed three times with 2 ml of ice-cold 10 mM phosphate-buffered saline, pH 7.0. Attached cells were scraped with a rubber policeman into 10 mM potassium phosphate, pH 7.0/5 mM magnesium sulfate/0.5 mM dithiothreitol/10% (vol/vol) glycerol and sonicated twice for 30 sec (Branson sonifier set at 50 W). Particulate material was removed by centrifugation at 100,000 X g at 40 for 1 hr. Total activity in high-speed supernatants was measured by using the coupled lactate dehydrogenase assay (18). The two different isozyme forms were measured by: (i) differential kinetic assay, based upon greater sensitivity of K-isozyme to inhibition by tryptophan (19) and (if) separation on DEAE-cellulose columns that, under the conditions used, bind only the L-isozymic form (20). Supernatant aliquots were applied to columns (10 X 0.6 cm), the K form was eluted with additional buffer, and the bound L form was eluted with buffer containing either 0.5 M KCl or with a 0-0.5 M KCl gradient. Enzyme activity and protein concentrations in co...

Kidney cortex pyruvate kinase was compared to heart and liver isoenzymes with respect to solubility in (NH4)2-S04, isoelectric points, sedimentation, and kinetic properties. These data suggest that two-thirds of the activity in kidney cortex extracts is a pyruvate kinase molecularly distinct from muscle or liver isoenzymes. The probable in vivo form of this enzyme appears to exist as an R,T conformational pair with mean p/ values of 6.77 and 6.41 and molecular weights of about 200,000. These conformers, whether in homogenates or partially purified, may be converted to forms of the enzyme with high Kq.ós values for phosphoenolpyruvate, molecular weights of about 100,000 and mean p/ values of 5.13, 7.50, or 7.70. The lower and higher molecular weight conformers were shown to be largely interconvertible. The remaining third of the activity in kidney cortex extracts had properties similar to a conformational variant of the major isoenzyme of liver.

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