Glycerokinase activity in human brown adipose tissue

Brown adipose tissue (BAT) is known to be responsible for heat production in newborn and adult hibernating mammals. In rats and mice, BAT has been demonstrated to possess a much higher glycerokinase activity than white adipose tissue (WAT). It has been speculated that this high activity may cause the futile cycle of triglyceride breakdown and resynthesis to be activated, thus contributing to heat production. However, at present very little information is available regarding the location, function, and quantitative importance of BAT in adult human subjects. Our objective in this study was to locate BAT in human subjects and to characterize it biochemically, especially with respect to the enzyme glycerokinase. We have looked for histologically identifiable BAT in 32 human subjects and found it in 12 subjects. Most of the BAT samples were obtained from perirenal adipose depots in children undergoing surgery. Some of the samples were almost totally comprised of BAT cells, whereas others were a mixture of BAT cells and WAT cells. The glycerokinase activity per gram of tissue was higher in BAT than in WAT in all the subjects where the above comparison was made. The activity per mg protein or per microgram DNA was higher in most BAT samples. In one pure BAT specimen, the basal lipolytic rate and the lipoprotein lipase activity were measured and they were both higher in BAT than in the WAT obtained from the same patient. These results show that human brown adipose tissue possesses an enzymatic profile very similar to that of rodent brown adipose tissue.