Comp. Biochem. Physiol. Vol. 103B, No. 1, pp. 101-104, 1992 0305-0491/92 $5.00 + 0.00 Printed in Great Britain © 1992 Pergamon Press Ltd GLYCEROLIPID BIOSYNTHESIS IN ADIPOSE TISSUE OF THE BOVINE DURING GROWTH MOHAMED BOUYEKHF,* DANIEL C. RULE*'~ and CHING YUAN HU* *Department of Animal Science, University of Wyoming, Laramie, WY 82071, U.S.A. Tel.: (307) 766-3404; Fax: (307) 766-5098; ~:Department of Animal Sciences, Oregon State University, Corvallis, OR 97331-6702, U.S.A. (Received 10 February 1992; accepted 11 March 1992) Abstract--1. Rates of palmitate esterification in tissue slices and glycerophosphate acyltransferase activity in homogenates were determined in bovine subcutaneous and intermuscular adipose tissue at 340, 418 or 498 kg of live weight. 2. Lower rib section fat accretion rates were observed from 340 to 418 kg than from 418 to 498 kg. 3. Changes in palmitate esterification rates at different body weights were consistent with reduced rib section fat accretion as well as with reported differences in fat accretion in subcutaneous and intermuscular fat depots. 4. Glycerophosphate acyltransferase activity was increased at 418 kg and remained elevated whereas palmitate esterification was decreased at 418 and then increased at 498 kg. 5. Differences between palmitate esterification and glycerophosphate acyltransferase in vitro may have been related to differences in substrate supply. INTRODUCTION Of the metabolic processes involved in accretion of lipid in adipose tissue in ruminants, glycerolipid biosynthesis could be considered the ultimate process because this anabolic pathway results in synthesis of triacylglycerol, the major storage form of lipid in adipose tissue. Effects of growth on glycerolipid biosynthesis in adipose tissue of rats (Jamdar and Osborne, 1981) and swine (Rule et al., 1989) have been investigated. Effects of growth in the bovine on glycerolipid biosynthesis have not been reported, Fatty acid biosynthesis rates, however, were shown to increase in bovine subcutaneous adipose tissue during growth (Pothoven et al., 1975; Whitehurst et al., 1981; Smith et al., 1984). Furthermore, lipogenesis was shown to decrease at the end of typical growth phases in adipose tissue of the bovine (Pothoven et al., 1975; Smith et al., 1984). Smith et al. (1984) also observed activities of several lipogenic enzymes to follow the same pattern of change as lipogenesis with growth in bovine adipose tissue. The purpose of this research was to determine rates of palmitate esterification (PE) and glycerophosphate acyltrans- ferase (GPAT) activity in two adipose tissue sites in beef cattle at 340, 418 and 498 kg of live weight. METHODS Reagents Carboxylate-14C palmitic acid [(NEC-075H) and L-(14C (U))]-glycerol-3 phosphate (NEC-608) were purchased from New England Nuclear (Boston, MA). Bovine serum albu- min (fatty acid-poor, No. 2295-00) was purchased from Armour Pharmaceutical Co. (Kankakee, IL) 60901. Sub- strates, cofactors, ingredients for buffers and incubation tTo whom correspondence should be addressed. media, and osmium tetroxide were purchased from Sigma Chemical Co. (St Louis, MO). Animals and diet Twenty steers weighing 267 + 2 (SEM) kg were randomly assigned to slaughter groups of six, seven and seven steers each. Average weight at slaughter of each group was (kg+__SE) 340+4 (N=6), 418+6 (N=7) and 498+6 (N = 7). Steers were ¼ each Pinzgauer, Red Poll, Hereford, and Angus. Steers initially received a diet consisting of 40% ground corn and 60% alfalfa hay with trace mineralized salt free choice. The proportion of corn was gradually increased to 60% by 30 days and then 65% corn by 60 days. After 90 days, all diets were changed to 70% corn, 15% corn silage and 15% alfalfa hay, and were unchanged during the remainder of the study. Tissue preparation and incubation All steers were sacrificed in the University of Wyoming abbatoir by stunning followed by exsanguination. Immedi- ately after exsangnination the hide was cut and subcu- taneous adipose tissue (SC) was removed from the loin edge anterior to the gluteus medius muscle and over the longis- simus dorsi. As soon as the hide was stripped from the round, intermuscular adipose tissue (IM) was removed from between semimembranosus and semitendinosus muscles. All samples were taken from the fight side of the carcass. Samples were placed in Krebs-Ringer bicarbonate buffer (KRB), pH 7.4 at 37°C and brought immediately to the laboratory which was located in the same building as the abbatoir. For PE, tissues were sliced 0.5 mm thick and 100-150 mg were incubated in 3 ml KRB that contained 10 mM glucose, 4.5% BSA, 1000/~U/ml bovine insulin, 1.0/aCi of ~4C- palmitate and 0.75 mM potassium palmitate. Incubations were conducted in 150 x 25mm screw-cap tubes flushed with 5% CO2 in oxygen at 37°C in a gyrorotary water bath (2.5 RPS). Incubations lasted 2 hr and were terminated by first rinsing tissue slices with a mixture of KRB:BSA:water (66:15:18, v/v/v, 37°C) then with a solution of KRB:water (66: 33, v/v, 37°C) to rid slices of excess isotope and BSA, 101