Technical note: Analysis of total lipid and triacylglycerol content in small liver biopsy samples in cattle

A procedure is described for analyzing total lipid (TL) and triacylglycerol (TAG) in 2 sequential steps using small amounts (<100 mg) of bovine liver tissue. The TL was measured gravimetrically and TAG was measured enzymatically in the TL extract, using an automated analyzer. For gravimetric TL determination in milligrams per gram of liver fresh weight (FW), TL was extracted from homogenized tissue samples with hexane:isopropanol (at 20 degrees C, 24 h, constant agitation). The routine method was modified by adding a second hexane extraction step to optimize lipid extraction. The dry lipid extract was dissolved in hexane and aliquoted according to TL content for TAG analysis. An extra incubation period of 16 h was included for complete hydrolysis of TAG, using microbial lipase and nonaethylene glycol monododecyl ether detergent, before TAG was measured enzymatically using commercial test kits. Triolein was used as an internal standard. Repeated TL analysis (n = 3) of liver specimens from 10 cows (range, 40 to 314 mg/g of FW) yielded a mean CV of 2.2%, whereas repeated TAG analysis (range, 4 to 260 mg/g of FW) yielded a mean intraday CV of 2.5% (n = 5) and a mean interday CV of 3.4% (n = 4). Intraday (n = 5) and interday (n = 4) CV for repeated TAG analysis in triolein standards were <1 and <3%, respectively. Recovery of TAG in triolein standards varied between 99 and 103%. In part 2 of the experiment, hepatic TL and TAG were measured in 150 German Holstein cows to verify the test method in a large sample size. For repeated hepatic TL (n = 3) and TAG (n = 5) determination, mean CV of <2.8 and <1.5%, respectively, were found. The proportion of TAG relative to TL increased linearly to a breakpoint of approximately 100 mg TL/g of FW, at which point it reached a plateau at approximately 68%, indicating an accumulation of other lipid fractions in hepatic tissue with hepatic TL above the breakpoint. Calculation of hepatic TAG from TL was reasonably accurate when a 2-slope linear broken-line model (r(2) = 0.98) was used. Above a TL of approximately 40 mg/g of FW, calculated TAG values deviated by only +/-15% from measured hepatic TAG.