Distinction of Different States of the Enzyme and the Possible Role of the Fat Cell in the Maintenance of Tissue

1. Incubation of intact epididymal adipose tissue from fed rats at 370 in an albumin solution at pH 7-4 in vitro results in a rapid loss of clearing-factor lipase activity until a low activity, stable to prolonged incubation, is attained. The clearing-factor lipase activity of intact tissue from starved rats, which is initially much less than that of tissue from fed rats, is mainly stable to incubation at 37°. 2. Much of the clearing-factor lipase activity of intact epididymal adipose tissue from fed rats is inactivated by collagenase. The enzyme activity of intact tissue from starved rats is not inactivated by collagenase. 3. The clearing-factor lipase activity of fat cells isolated from the epididymal adipose tissue offed rats is stable to prolonged incubation at 37°. It represents only a small proportion of the total activity of the intact tissue. In starved rats, the isolated fat cells contain a much higher proportion of the activity of the intact tissue. Their activity is also stable at 37°. 4. Incubation of isolated fat cells in a serum-based medium leads to a progressive rise in clearing-factor lipase activity. Actinomycin increases the extent of this rise in activity. No rise in clearing-factor lipase activity occurs when stromal-vascular cells isolated from epididymal adipose tissue are incubated in the medium. 5. The findings indicate that less than 20% of the activity of intact adipose tissue from fed rats is retained when fat cells are isolated from the tissue by collagenase treatment. The activity that is lost could be that which normally functions in the uptake of triglyceride fatty acids by the tissue.