Mitochondrial Lysine Uptake Limits Hepatic Lysine Oxidation in Rats Fed Diets Containing 5, 20 or 60% Casein

Manuscript received 1 April 1998. Initial reviews completed 7 August 1998. Revision accepted 31 August 1998.

Kenneth P. Blemings^*, , Thomas D. Crenshaw^*, and Norlin J. Benevenga^{*, ,}

Departments of ^* Department of Animal Sciences, University of Wisconsin Madison, Madison, Wisconsin 53706, USA and  Department of Nutritional Sciences, University of Wisconsin Madison, Madison, Wisconsin 53706, USA

Sixty male Sprague-Dawley rats were randomly allotted to receive diets containing 5, 20 or 60% casein. Rats had access to the diet only during the initial 8 h of the daily 12-h dark period. Hepatic mitochondrial lysine uptake, lysine -ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SacD) activities, and in vitro lysine oxidation (LOX) were measured 0, 6, 12, 18 and 24 h after the start of the dark period. Diurnal variation of mitochondrial lysine uptake was not detected (P > 0.10) although uptake varied 3-fold over 24 h. Mitochondrial lysine uptake was greater (P < 0.05) for rats fed diets containing 60% casein than for rats fed diets containing 5% casein. Diurnal variation of LKR was detected (P < 0.05) in rats fed diets containing 20 and 60% casein. Diurnal variation of SacD was detected (P < 0.05) in rats fed diets containing 60% casein. Increased casein consumption resulted in increased LKR and SacD activities (4- to 5-fold; P < 0.05). Diurnal variation of LOX was detected in rats fed diets containing 20 and 60% casein (P < 0.05). Increasing the casein concentration in the diet from 5 to 60% resulted in a 7-fold increase in LOX (P < 0.05). To make rate comparisons, LKR and SacD activities and LOX were predicted from a range of substrate concentrations (0.1 to 5.0 mmol/L). Overall, LKR and SacD were 6-107 times that of LOX, suggesting that, in liver, mitochondrial lysine uptake limits LOX.

The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2427-2434
Copyright ©1998 by the American Society for Nutritional Sciences

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