35 y/o man with 3 yr history of enlarging nose, coarse facies, muscle weakness, increased hand/foot size. Large fleshy nose and prognathism on exam. High IGF-1 in serum. MRI shows pituitary adenoma. Morphologic analysis of the tumor shows a densely granulated somatotroph adenoma. Further studies show that the G alpha-s subunit of the tumor G proteins lack GTPase activity. The tumor cells most likely have an increased activity of which enzyme?