Relationship of plasma leptin concentration to intramuscular fat content in beef from crossbred Wagyu cattle

Plasma leptin concentrations and beef cattle carcass characteristics in eight Continental Crossbred steers [0% Wagyu Cattle (WC)] were compared to crossbred cattle with 50 and 75% WC (eight steers each) genetic makeup to determine if a relationship exists between plasma leptin concentrations and intramuscular fat content (marbling) in beef cattle. Plasma leptin concentrations were measured at two stages of cattle growth, 16 and 4 wk prior to slaughter (W P S). Beef cattle characteristics including marbling score, ribeye area, i.m. total lipid content, and backfat depth were determined, and correlation coefficients obtained between these traits and leptin concentration at both sampling dates. Plasma leptin concentrations increased relative to the lipid content in the 24 steers based on the significant positive correlation observed between plasma leptin and total lipids (% wet weight) from both pars costalis diaphragmatis (p.c.d.)(16 WPS: r = 0.69, P = 0.0004; 4 WPS: r = 0.35, P = 0.104) and longissimus (16 WPS: r = 0.59, P = 0.002; 4 WPS: r = 0.51, P = 0.011) muscles. A trend was observed, however, at 4 WPS when the groups of varying Wagyu genetics were compared. Plasma leptin was positively correlated with muscle lipid content for the 0% Wagyu cattle (longissimus: r = 0.62, P = 0.103; p.c.d.:r = 0.40, P = 0.410)but there was almost no correlation in these parameters for the 50% WC (longissimus: r = 0.11, P = 0.797; p.c.d.: r = 0.005, P = 0.990). Plasma leptin concentration was negatively correlated with lipid content in the 75% WC (longissimus: r = –0.60, P = 0.120; p.c.d.: r = –0.65, P = 0.164). The results suggest that increasing Wagyu genetics negates any relationship between leptin concentrations and i.m. fat content in cattle. Key words: Wagyu crossbred cattle, meat quality, intramuscular fat, marbling, leptin

Characterization of microsomal diacylglycerol acyltransferase activity from bovine adipose and muscle tissue

The activity of the triacylglycerol bioassembly enzyme, diacylglycerol acyltransferase (DGAT), was characterized in microsomal fractions prepared from bovine subcutaneous (SC) adipose, intramuscular (IM) adipose, and muscle (pars costalis diaphragmatis) tissue. The activity of DGAT was generally higher from SC adipose tissue than from IM adipose or muscle tissue. The characteristics of DGAT activity from the three bovine tissues resembled the activity characteristics observed in previous studies from various other organisms and tissues; the pH optimum was near neutrality, the activity was almost completely inhibited by pre-incubation with N-ethylmaleimide (NEM), and the enzyme accepted a broad range of acyl-CoAs and sn-1,2-diacylglycerols. In some aspects, the SC adipose tissue DGAT activity was different from the DGAT activity from the other two tissues. The SC adipose tissue DGAT activity was not as susceptible to inhibition by NEM as the enzymes from the two other tissue sources, and it exhibited increased specificity for substrates containing oleoyl moieties. The differences in DGAT properties between the three bovine tissues may account to some extent for the differences in the relative fatty acid composition and the positional distribution of fatty acids in triacylglycerol between bovine tissues. The observed differences in enzymatic properties also support recent biochemical and molecular genetic observations that imply the existence of multiple DGAT genes and/or isoforms.

Human acylation stimulating protein enhances triacylglycerol biosynthesis in plant microsomes

Diacylglycerol acyltransferase has a universal role in catalyzing the acyl-CoA-dependent formation of triacylglycerol in microorganisms, animals and plants. Acylation stimulating protein, from human blood, is known to enhance diacylglycerol acyltransferase activity and triacylglycerol biosynthesis in human adipocytes. In the current study, acylation stimulating protein was also shown to enhance diacylglycerol acyltransferase activity in microsomes from cell suspension cultures of oilseed rape. Enzyme stimulation occurred over the pH range of 6-9 but the degree of stimulation decreased with increasing ionic strength at pH 7.4. Varying acyl-CoA concentration did not affect the degree of stimulation. Membranes from triacylglycerol producing cells in plants and humans may have similar binding sites for acylation stimulating protein which have been preserved during molecular evolution. The results suggest that human acylation stimulating protein may be useful in modifying lipid biosynthesis in plants.

Relationship of fatty acid composition to intramuscular fat content in beef from crossbred Wagyu cattle

The deposition of i.m. fat, or marbling, in cattle is recognized as a desirable carcass trait in North American beef grading schemes. In order to investigate the relationship between degree of marbling and fatty acid composition of whole bovine muscle, we extracted the total lipid from pars costalis diaphragmatis (PCD) (n = 23) and longissimus (n = 36) muscles from Wagyu crossbred cattle that were assigned Canadian Grading Agency marbling scores ranging from 1 to 8 on an inverse 10-point scale (i.e., a score of 1 indicated "very abundant" marbling and a score of 10 would be assigned to a carcass "devoid" of marbling). Fatty acid methyl esters (FAME) of the total lipid and triacylglycerol fractions were resolved and quantified through GLC. Marbling scores were negatively associated with total lipid from both PCD (r = -.57, P < .01) and longissimus (r = -.80, P < .001). Differences between PCD and longissimus were found for almost all FAME studied from both lipid fractions, but no differences (P > .05) were seen when the monounsaturated:saturated fatty acid (MUFA/SFA) ratios were compared. Heifers had higher (P < .05) oleic acid content and lower (P < .05) palmitic acid content in lipid extracted from both muscles, resulting in higher (P < .05) MUFA/SFA ratios than those for steers. The relative amount of myristic acid increased as the lipid content (total lipid and triacylglycerol) increased in either longissimus (r values from .48 to .55; n = 36; P < .01) or PCD muscles (r from .67 to .76; n = 23; P < .001). The relative amount of linoleic acid (cis-9, cis-12 isomer) from total lipid was negatively associated with all chemical measurements of lipid from the longissimus (r from -.52 to -.64; n = 36; P < .001) and PCD muscles (r from -.75 to -.85; n = 23; P < .001). This association was not significant (P > .1) for either muscle when linoleic acid from the triacylglycerol fraction was examined, suggesting the negative association between this fatty acid and lipid content was due to a dilution of membrane phospholipids with increasing triacylglycerol. Indices of fatty acid elongase activity, calculated from FAME data, implicated the balance between this enzyme activity and fatty acid synthase as a source of variation between animals displaying various degrees of marbling and worthy of further investigation to better understand the process of marbling fat deposition in beef cattle.

Evaluation of diacylglycerol acyltransferase as an indicator of intramuscular fat content in beef cattle

The development of a biochemical or molecular marker for predicting the propensity for cattle to deposit intramuscular (i.m.) fat would reduce the time required to improve for this trait. In the current study, diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) activity was investigated as a potential biochemical marker for predicting the deposition of i.m. fat. The activities of DGAT were determined in microsomal fractions of subcutaneous (s.c.) adipose tissue, i.m. adipose tissue and muscle tissue sampled from Wagyu crossbred cattle at slaughter weight. The i.m. adipose and muscle tissues were dissected from the pars costalis diaphragmatis (p.c.d.) muscle. The lipid content (LC) of the p.c.d. muscle correlated with the LC of the longissimus muscle (r = 0.51; P < 0.05). There was an inverse correlation between p.c.d. i.m. adipose tissue DGAT specific activity and LC of the p.c.d. (r = −0.50; P < 0.05) suggesting that DGAT activity was down-regulated as i.m. adipose tissue became filled. There was a strong correlation (r = 0.74; P < 0.001) between DGAT total activity in i.m. adipose tissue and muscle DGAT total activity, suggesting DGAT activity is coordinately regulated in these two tissues. Key words: Bovine, intramuscular fat, diacylglycerol acyltransferase, Bos taurus