Our long-term hypothesis is that hepatic oxidation of acetyl CoA is stimulated by anaplerosis of the tricarboxylic acid (TCA) cycle by propionate, causing an increase in hepatic energy charge. The objective of this study was to determine potential bottlenecks associated with propionate metabolism. Six cows (4 to 18 d postpartum) were used in a crossover design (3 d with 1 d rest). Cows were blocked from feed 1 h before treatment and received a pulse dose to the rumen of either 500 mL of water (control) or 2.0 moles of propionic acid (PA) in a 500 mL solution. Plasma and liver samples were collected immediately before dosing (T0) and at 10 (T10) and 20 (T20) min post-dosing. Liver samples were analyzed for acetyl CoA (A-CoA), propionyl CoA (P-CoA), methylmalonyl CoA (M-CoA), succinyl CoA (S-CoA), succinate, fumarate, and malate and plasma was analyzed for propionate. Samples were standardized relative to T0 (T0 = 0). Data were analyzed with the Proc Mixed procedure in SAS (v.9.4). The PA treatment increased plasma propionate at T10 compared with control but propionate declined rapidly (interaction P = 0.04). PA tended to decrease A-CoA, did not affect P-CoA, tended to increase M-CoA and S-CoA, and increased succinate, fumarate and malate. Although interactions of treatment and time were not detected for liver metabolites, M-CoA and S-CoA were greater at T20 than T0 (P = 0.01 for both) and numerically greater than T10 indicating possible bottlenecks for metabolism of M-CoA and S-CoA. M-CoA requires vitamin B₁₂ and its supplementation may improve efficiency of propionate metabolism by alleviating that bottleneck. Our research identified M-CoA and S-CoA as possible metabolic bottlenecks to examine in future work. Table 1.

Metabolite	Control			2 mol of PA		SE	P-value
	T10	T20		T10	T20		Trt	Time	Trt × Time
Plasma propionate	−0.001	0.170		5.690	1.545	1.111	<0.01	0.05	0.04
A-CoA	0.006	0.214		−0.475	0.063	0.202	0.08	0.04	0.341
P-CoA	1.232	0.206		1.228	0.271	0.779	0.96	0.14	0.96
M-CoA	−0.163	−0.191		0.265	1.655	0.588	0.07	0.27	0.25
S-CoA	−0.223	−0.319		0.132	1.406	0.502	0.06	0.26	0.19
Succinate	−0.048	0.004		0.247	0.488	0.152	<0.01	0.14	0.32
Fumarate	−0.021	−0.162		0.419	0.349	0.149	<0.01	0.49	0.81
Malate	0.013	−0.130		0.479	0.410	0.188	0.01	0.54	0.83