1220 Effects of a modified stair - step compensatory growth model for gestating beef

The objective was to look at the activity of the hepatic cobalamindependent enzyme MCM in dairy cow during the transition period. This enzyme controls the utilisation of methylmalonyl-CoA for neoglucogenesis and thus might be strongly stimulated in dairy cow by the intake of transition and early lactation diets and the increased needs in glucose for milk production. Liver biopsies were taken up repeatedly from 6 multiparous cows 3 wk before calving (-3 wk) and at 2, 4 and 8 wk of lactation. MCM activity was assayed spectrophotometrically from purified mitochondrial matrix. MCM activity decreased from 14.04 ± 1.29 to 7.58 ± .75 nmol/min/mg of intramitochondrial proteins (P < .037) between -3 wk and 2 wk of lactation then returned (P < .004) and stayed at its initial level at wk 4 and 8. Apparent Km and Vmax values towards methylmalonyl-CoA (mean initial values : 175.2 ± 34.0 μM and 19.96 ± 3.16 nmol/min/mg of protein, respectively) followed the same quadratic pattern (P < .052). The dose-response curves of MCM activity towards its cofactor adenosylcobalamin (AdoCbl) indicated the same variations around calving for the apomutase but not for the holomutase part of the activity which remained stable until wk 2 then increased from 4.95 ± .45 to 8.00 ± .80 nmol/min/mg of protein at wk 8 (P < .023). Holomutase activity represented 41.2, 65.3, 61.5 and 59.6 % of total MCM activity at -3, 2, 4 and 8 wk, respectively. This increase was associated to a reduction in plasma vitamin B12 concentrations (-33 %, P < .003). Apparent Vmax values towards AdoCbl (initial level 8.59 ± .71 nmol/min/mg of protein) varied also quadratically. Apparent Km values towards AdoCbl were constant until wk 2 (.58 ± .20 to .69 ± .14 μM), decreased by 36 % at wk 4 (P < .056) and then returned to the inital values at wk 8 (P < .001). Our results show that MCM activity is decreased during the first weeks of lactation and consequently might reduce neoglucogenesis.