SummaryThe rate of histone phosphorylation in adipose tissue abstracts obtained from adults and newborns by cyclic adenosine mono-phosphate (cAMP)-dependent protein kinase was studied. Aden-sine triphosphate (ATP) had an apparent Km of 0.18 μM. Further kinetic studies indicated that human white adipose tissue is similar to brown adipose tissue of newborn rats in that it probably contains more than one type of soluble cyclic AMP-dependent protein kinase activity.Cyclic AMP activated the human enzyme with maximum velocity reached at 1μM concentration. Cyclic guanosine monophosphate, however, produced only a low level activation of the enzyme, perhaps more dose-dependent when arginine-rich histone served as the phosphate acceptor.The activity in samples of subcutaneous adipose tissue of two age groups of healthy newborns was compared with samples obtained from adult volunteers. Both the basal activity and the cAMP-activated enzymes showed an increase with age. The velocity values for the rate of phosphorylation observed in the presence of relatively high NaCl concentrations were depressed by approximately 50%; the “activity ratio” under these conditions, however, reflected the truein vivostate of activation of the type II protein kinase.It has been confirmed by a series of methodologic experiments that the cyclic AMP dependent protein kinase of adult human white fat exhibits the same properties as the type II enzyme described in rat white adipose tissue. The activity ratio was determined on samples of subcutaneous adipose tissue of different age groups of healthy human newborns and of adult volunteers. The system is highly activated immediately after birth (less than 12 hr) and a subsequent decrease (12–48 hr) in the level of activation is observed.SpeculationThe protein kinase system, its composition, characteristics, and in particular, its level of activation under physiologic and pathologic conditions is of utmost importance in our understanding of the molecular mechanisms of hormonal regulation occurring in infant adipose tissue. Experimental conditions described here allow determinations of the in vivo level of activation of the cyclic AMP-dependent system and after further modification may help to link specific hormonal stimulation with phosphorylation, and, thus, functional changes, occurring in specific intracellular proteins.