NUTRITIONAL REGULATION OF THE ACTIVITIES OF LIPOGENIC ENZYMES OF RAT-LIVER AND BROWN ADIPOSE-TISSUE

Nutrition-induced effects on the activity of enzymes of lipogenesis, f atty acid synthase (FAS; EC 2.3.1.85), ATP citrate lyase (ACL; EC 4.1. 3.8), malic enzyme (ME; EC 1.1.1.40), glucose-6-phosphate dehydrogenas e (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (PGDH; EC 1.1.1.44) were investigated in liver and interscapular brown adipose t issue (BAT) of rats. The lipogenic enzymes could be grouped into two c ategories according to their response to dietary manipulations: FAS an d ACL, both key enzymes of lipogenesis, responded fast and strongly to dietary manipulations. ME, GGPDH and PGDH, enzymes which also contrib ute to metabolic pathways other than lipogenesis, responded in a more sustained and less pronounced fashion. Feed deprivation caused the spe cific activities of lipogenic enzymes to decline several-fold Refeedin g of previously fasted (up to 3 days) animals increased the activities dramatically (10-to 25-fold) to far above pre-fasting levels (''overs hoot''). Repetition of the fasting/refeeding regimen increasingly impa ired the ability of both tissues to synthesize overshooting enzyme act ivities in the subsequent refeeding period. The fasting-induced declin e of the activities was prevented when sugars were provided to the ani mals via drinking water. The sugars displayed different effectivities: sucrose = glucose > fructose > maltose >> lactose. Sugars as the sole nutrient after fasting were also able to induce overshooting enzyme a ctivities. Again, activities of FAS and ACL responded in a more pronou nced fashion than the other three enzymes. Transition from feeding one diet to feeding a new diet of different composition led to adaptation of the lipogenic enzyme activities to levels characteristic for the n ew diet. Replacing a low-carbohydrate with a high-carbohydrate diet pr oceeded with major alterations of enzyme activities. This process of a ttaining a new level took up to 20 days and involved pronounced oscill ations of the specific activities. In contrast, when a high-carbohydra te diet was replaced with another diet, particular one high in fat, tr ansition to new enzyme activities was completed within 2-3 days and pr oceeded without oscillations. All dietary manipulations caused more pr onounced responses in young (35d-old) than in adult (180d-old) animals .