Degradation of two soluble proteins--casein and egg protein by a macro in vitro method

Degradation of casein and egg protein was studied with whole rumen contents (RC) in a macro in vitro system to elucidate previous findings of initial rapid disappearance of soluble proteins in vitro. Five to 7.5 kg of RC from a dry and/or a lactating cow were incubated with buffer and casein or egg protein for 180 min with frequent sampling. Degradation was measured as loss of trichloroacetic acid precipitable N (TCA-N) from the inocula. Normal (39°C) and low (2°C) temperature incubations were examined in Exp. 1, using 1 g of TCA-N from casein. Four levels of casein (0-12 g TCA-N) in Exp. 2 and four levels of egg albumin (0-24 g TCA-N) in Exp. 3 were fermented at 39°C. Initial recovery of casein TCA-N was 106% at 2°C and 56% at 39°C (Exp. 1). Casein (TCA-N) recovered initially increased in Exp. 2 from 21% at 3 g to 86% at 12 g TCA-N, while absolute loss remained relatively constant at 358 mg TCA-N/kg RC (SD=47). Fractional degradation rate was highest (0.03/min) at the intermediate dosage level. In the absence of rumen fluid (Exp. 4), no casein was lost. Initial egg protein recovery was on average 103% (Exp. 3). Recovery seemed unaffected by dosage level, and absolute degradation rate was relatively constant over time and increased with dosage level (p<0.001) from 1.48 to 2.95 mg TCA-N/(kg RC × min). Maximum degradation rate [mg TCA-N/(kg RC × min)] and affinity constant (mg TCA-N/kg RC) were estimated at 261 and 1650, respectively. It is concluded that a surprisingly constant amount of casein disappears immediately from warm rumen fluid and that this does not occur either with chilled RC, in the absence of rumen fluid, or when replaced with egg protein. The mechanisms for this disappearance are yet to be discovered.