Adrenaline-sensitive adenylate cyclase of human fat cell ghosts: properties and hormone-sensitivity.

Abstract

Abstract. Some of the properties of the human fat cell adenylate cyclase were studied in fat cell ghosts prepared from subcutaneous adipose tissue of twenty‐three patients. Under standard assay conditions (1 mmol/l ATP, 5 mmol/l mg2+, pH 8.5), the basal activity ranged from 0.6 nmol to 4.4 nmol of 3′,5′‐cyclic AMP formed per mg protein per 15 min.Peptide hormones (ACTH, glucagon) were ineffective in stimulating adenylate cyclase. Adrenaline, however, induced a 1.5–5‐fold enhancement of 3′‐5′‐cyclic AMP formation.The rates of basal and hormone‐stimulated enzymic activity were maximal at pH 8.5. The relative extent of hormonal stimulation, however, was greater at pH 7.4.ATP in excess of magnesium had an inhibitory effect on basal and hormone‐stimulated enzymic activity. Magnesium in excess of ATP caused a concentration‐dependent increase of both expressions of enzymic activity.Of the guanine nucleotides, GTP (0.1 mmol/l) augmented hormone sensitivity via a preferential inhibition of basal activity. 5′‐Guanylyl‐imidodiphosphate, however, caused a 2–5‐fold increase of basal and adrenaline‐stimulated activities.The results show that human fat cell ghosts contain an adrenaline‐sensitive adenylate cyclase system, whose hormone sensitivity is profoundly influenced by ATP, magnesium ions, pH, and guanine nucleotides. Artificial cofactors such as 5′‐guanylyl‐imidodiphosphate or α‐blocking agents are not essential for the expression of hormone sensitivity.