Relation of Phospholipids to Selected Tissue Components in Light and Dark Portions of Porcine Semitendinosus Muscle

The semitendinosus of pork contains a light and a dark portion that have physicochemical properties similar to those of uniformly white and red muscles, respectively. Although the two portions differ in metabolic activity they function as one muscle. The purpose of this study was to investigate the relation of phospholipid concentration to predominant fiber of the two portions of the porcine semitendinosus. The light and dark portions of the porcine semitendinosus were selected in order to minimize the possible effect of exercise on phospholipid content.

Phospholipids of the light and dark tissue of the porcine semitendinosus were studied in relation to total nitrogen (N), selected nitrogenous components, DNA and total lipid of the tissue. Phospholipids were separated into lecithin, cephalin and sphingomyelin fractions by thin-layer chromatography, and the percentage of each fraction was estimated by densitometry.

Semitendinosus light tissue contained more total and protein N than the dark. Sarcoplasmic and nonprotein N were higher in the light than dark portion on the basis of mg per g of tissue. Fibrillar and stroma. N were similar in the two portions. No differences between light and dark portions in any of the nitrogenous compounds expressed as percent of total N were found.

Average DNA content was higher in the dark tissue than light regardless of means of expressing concentration. A wide variation among the animals was found.

The dark portion contained more moisture and less lipid than the light portion. Phospholipid content was higher in the dark than light tissue whether expressed in terms of wet weight, dry weight, nonlipid dry weight, total N or protein N of the tissue. No difference was found in the concentration of lecithin, cephalin or sphingomyelin fractions of the two portions. Lecithins accounted for about 60 percent of the total of the three fractions, cephalins for about 30 percent and sphingomyelin for about 10 percent.

Phospholipid content per unit weight of DNA was slightly higher on the average for the light portion than the dark, but the difference only approached significance. Further investigations of phospholipid-DNA ratio in red and white muscles would be of value if a more reproducible method for DNA determination were available.

From the results it would appear that the phospholipid content of the light and dark portions of the porcine semitendinosus muscle was related more closely to metabolic activity, as indicated by fiber content, than to physical exercise.