Effects of diphenyliodonium chloride on proteolysis and leucine metabolism by rumen microorganisms

Diphenyliodonium chloride (DIC) at 0.3 and 0.8 mM inhibited hydrolysis of casein by a rumen microbial ecosystem unadapted to DIC by as much as 30%. Adaptive effects to DIC on leucine oxidation over 37 days were evaluated in a batch culture system repetitively diluted with culture media devoid of protein but containing carbon and nitrogen sources essential for growth. DIC concentrations up to 0.1 mM had no effect on concentrations of microbial protein during the 37 day period of semi-continuous culture. Concentrations of ammonia were depressed (P ≤ 0.05) by day 5 after initiating an influx of 0.1 mM DIC while other levels of DIC were without effect. Effects of DIC on oxidation of leucine were determined in aliquots of removed culture to which [1-14C]-leucine was added. Leucine oxidation was assessed as the 14CO2 evolved during a 1 h fermentation with [1-14C]-leucine and subsequent peroxide oxidation of the residual [1-14C]-α-ketoisocaporate (KIC). The proportion of 14CO2 recovered during fermentation and from KIC was essentially equal (regression coefficient of 1.0244 ± 0.04) and did not differ (P > 0.05) due to level of DIC or days of culture. The results suggests that leucine was deaminated via oxidative deamination to KIC followed by oxidative decarboxylation of KIC to isovalerate. These results illustrate that DIC can completely inhibit leucine degradation without detrimental effects on microbial growth in culture systems derived from rumen fluid that are allowed to adapt to DIC.