Stimulation of tumor growth in adult rats in vivo during an acute fast.

These experiments investigate an increase in tumor growth that occurs in adult rats in vivo during an acute fast. The effects of feeding, fasting, and underfeeding on the growth of Morris hepatomas 5123C and 7288CTC in Buffalo rats and of Walker carcinoma 256 and Jensen sarcoma in Sprague-Dawley rats were studied. Animals were matched for tumor size and growth during a period of ad libitum feeding preceding the fasting or underfeeding. Tumor growth was documented by increased size and incorporation of [methyl-3H]thymidine into tumor DNA. Fasting increased the rate of growth of the tumors 3 to 4 times over that measured in fed rats. This effect began during the first day of fasting and ended abruptly on refeeding. After refeeding tumor growth slowed to the rate in fed rats. Tumors from fed or fasted rats were not different in cellularity or dry weight/g wet weight. A positive growth response in the tumor required lipolysis and ketosis in the host. No stimulation was observed during an acute fast in either immature rats or in mature rats whose weights had been reduced by underfeeding. These animals have small fat stores and show no increase in arterial blood free fatty acid or ketone body concentrations during an acute fast. Finally, underfeeding of adult rats raised the blood concentrations of these nutrients to values that were intermediate between those in fasted and fed rats. Tumor growth rates in these rats were intermediate between those in fasted and fed rats. The results support the proposal that an increase in availability of free fatty acids and/or ketone bodies is the stimulus that increases the rate of tumor growth during an acute fast.

Mitochondrial malic enzymes. Purification and properties of the NAD(P)-dependent malic enzyme from canine small intestinal mucosa.

An NAD(P)-dependent malic enzyme with a specific activity of 40.6 mumol of NADH/min/mg of protein and an isoelectric point of 5.4 was purified to apparent homogeneity from canine small intestinal mucosal mitochondria. The purification procedure employed ammonium sulfate fractionation, Sepharose CL 6B gel filtration, chromatography on DEAE-cellulose to remove the interfering malate dehydrogenase, and affinity chromatography on 2',5'-ADP-Sepharose and NAD-agarose to take advantage of the dual coenzyme specificity. Antibody prepared from the purified enzyme produced a single peak upon cross-rocket immunoelectrophoresis against the mitochondrial sonicate. Continuous polyacrylamide gel electrophoresis showed NAD and NADP activity co-migrating with the native protein band. A single band of protein having an apparent Mr = 62,000 was seen on sodium dodecyl sulfate electrophoresis. At pH 7.3, gel filtration revealed a single peak of activity with NAD and NADP corresponding to an apparent Mr = 282,000. Gradient gel polyacrylamide electrophoresis at pH 9.0 indicated an additional broad band of activity corresponding to a Mr = 141,000. Under physiological conditions therefore the protein appears to exist as a tetramer of Mr = 282,000 composed of four equal subunits, whereas at elevated pH values during electrophoresis, partial dissociation to a dimeric species occurs.

Identification of an NAD(P)+-dependent 'malic' enzyme in small-intestinal-mucosal mitochondria.

An NAD(P)+-dependent 'malic' enzyme is shown to be present in mitochondria from small-intestinal mucosa. The intracellular location, activity and regulatory kinetic properties of the enzyme suggest that it participates in the major energy-producing pathway for net oxidation of glutamine-derived tricarboxylic acid-cycle intermediates.

Mitochondrial malic enzymes. An association between NAD(P)+-dependent malic enzyme and cell renewal in Sprague-Dawley rat tissues.

The activities of the mitochondrial NAD(P)+- and NADP+-dependent malic enzymes were measured in 11 tissues of the male Sprague-Dawley rat. The NAD(P)+-dependent malic enzyme was present in small intestinal mucosa, spleen, thymus, lung, and testis. Each of these tissues contain cells that are undergoing active rates of renewal. The NADP+-dependent malic enzyme was not confined to tissues undergoing cell renewal, and was present in mitochondria from brain, skeletal, and heart muscle, kidney, and lung and testis. Both enzymes were absent or at a low activity in normal and regenerating liver. The results support, and extent to nonneoplastic tissues, our proposal that cells which show active and sustained rates of renewal contain the NAD(P)+-dependent malic enzyme and have a unique intramitochondrial pathway for malate oxidation (Sauer, L.A., Dauchy, R.T., Nagel, W.O., and Morris, H.P. (1980) J. Biol. Chem. 255, 3844-3848).