Activation of adenylate cyclase in $\text{Acanthamoeba}$$\text{palestinensis}$

Abstract

Preincubation of $\text{Acanthamoeba palestinensis}$ homogenates in 0.25M sucrose-TM (2mM MgSO₄ and 5mM Tris-HCl, pH 7.4) at O°C for increasing periods of time up to 3 h, leads to a progressive increase in the activity of adenylate cyclase. In contrast, preincubation of i isolated membrane fractions enriched in enzyme activity in the same medium results in no activation. However, preincubation of membrane fractions in medium containing a high density of sugars (sucrose, glucose or fructose) mimics the activation obtained with homogenates. The high density sugar activation is time and temperature dependent, and reversible upon return to a low density medium. The high osmotic pressure of the sugars utilized may be a factor, since high concentrations of the sucrose polymer, Ficoll, which has low osmotic activity, causes not activation. Soluble activators, protein synthesis and changes in cyclic nucleotide phosphodiesterase activity were all eliminated as possible effectors of the apparent activation of adenylate cyclase. In contrast to mammalian adenylate cyclase, the endoplasmic reticulum localized enzyme of $\text{Acanthamoeba}$ is inhibited by NaF and is unaffected by GTP, adenosine, epinephrine, prostaglandin E₁, propranolol, and meclofenamic acid. These data indicate that the adenylate cyclase of $\text{Acanthamoeba}$ is structurally different from that of most mammalian cells.