Selective hormonal control of myo-inositol biosynthesis in reproductive organs and liver of the male rat.

Abstract

myo-Inositol biosynthesis has been examined in hypophysectomized and thyroidectomized male rats. After hypophysectomy, inositol-l-phosphate synthase [IL-myo-inositol-lphosphate lyase (isomerizing), EC 5.5. 1.4] Among the many tissues capable of synthesizing myo-inositol, the testis ranks as the richest mammalian source of inositol-1-phosphate synthase [lL-myo-inositol-l-phosphate synthase, EC 5.5.1.4], the NAD+-dependent enzyme that isomerizes glucose 6-phosphate to myo-inositol 1-phosphate (1) referred to here as simply inositol 1-phosphate. Because the testis is the target organ for various hormones and is itself the site of androgen biosynthesis, we were led to inquire into possible hormonal control of inositol biosynthesis. Although there have been sporadic reports on the effects of various physiological and pathological states on the levels of free inositol and inositol-l-phosphate synthase in certain tissues (2-7), there has been no systematic study of hormonal control of the enzyme. Here we report the effect in male rats of hypophysectomy, adrenalectomy, and thyroidectomy on tissue levels of inositol-l-phosphate synthase and preliminary experiments in the reversal of these effects by gonadotropic and thyroid hormones. MATERIALS AND METHODSMale Sprague-Dawley rats weighing approximately 200 g were obtained from Taconic Farms (Germantown, NY) and maintained as follows. Normal animals received regular tap water; hypophysectomized, 5% dextrose; thyroidectomized, 1% calcium lactate; and adrenalectomized, isotonic saline. Sham-operated rats were given the same fluids; all animals received regular laboratory chow ad lib. Tissues for enzyme assay were removed and weighed immediately after killing by decapitation. Seminal vesicles were washed with cold isotonic saline after removal of seminal fluid through an incision. Brain, liver, spleen, thymus, pancreas, kidney, testis, and ventral prostate were homogenized in a Potter-Elvehjem homogenizer with 2-3 vol of 0.154 M KCI/0.2 mM dithiothreitol. Epididymis and seminal vesicles were minced and then homogenized in a Polytron homogenizer with 3-5 vol of the same solution. The homogenates were centrifuged 20 min at 27,000 X g, and the supernatants were heated at 60'C for 2 min exactly, chilled in ice, and centrifuged at 105,000 x g for 1 hr. Solid ammonium sulfate was added to 40% of saturation (0.226 g/ml). The precipitate was collected by centrifugation, dissolved in 0.6-0.7 ml of 50 mM Tris acetate, pH 7.4/0.2 mM dithiothreitol and dialyzed against the same buffer overnight. The suspension was freed of insoluble material by centrifugation and assayed for inositol-1-phosphate synthase by incubation with glucose 6-phosphate (5 mM) and NAD' (1 mM); the resulting inositol 1-phosphate was determined as inorganic phosphate by the periodate method of Barnett et al. (8). Protein was determined by the method of Lowry et al. (9), with correction for the effect ofTris.

The Structure of the 1l-myo-inositol-1-phosphate Synthase-NAD+-2-deoxy-d-glucitol 6-(E)-Vinylhomophosphonate Complex Demands a Revision of the Enzyme Mechanism

“…Though large quantities of inositol are available from the diet, significant biosynthesis of inositol has been detected in organs where a significant blood barrier exists, such as the testes and brain (5)(6)(7)(8)(9). In fact, reduction of the brain inositol pool by inhibition of myo-inositol (MI) 1 monophosphatase has been suggested to be the mode of action for lithium in the treatment of bipolar disorder (10 -13).…”

Luminal secretion of myo-inositol by the rat epididymis perfused in vitro

“…In other studies there was also no synthesis of inositol from glucose (Eisenberg & Bolden, 1964) Robinson & Fritz (1979) exceeds the activity of phosphoglucose isomerase 12-fold (Brooks, 1976a), using the epididymal protein content given by Brooks (1976b), so loss of tritium from the Ci-position of glucose 6-phosphate during glycolysis (Hammerstedt, 1975) should be minimal, although the conflicting reports on the activity of the relevant enzymes in rat epididymal tissue (Robinson & Fritz, 1979;Maeda & Eisenberg, 1980;Hasegawa & Eisenberg, 1981) need clarification and bear directly on the results of this study. The observations may also relate to the presence of endogenous pools of metabolites which were not equilibrated during the experiments, although work on tissue slices indicates that intracellular glucose is very low in glucose-containing medium (Brooks, 1979).…”

“…The extent of rajO-inositol synthesis from glucose by the epididymis is debated (Eisenberg & Bolden, 1964;Robinson & Fritz, 1979;Maeda & Eisenberg, 1980;Hasegawa & Eisenberg, 1981) and although the potential exists, the contribution from luminal spermatozoa is not established (Roberts, Scouten & Nyquist, 1976;Loewus, Wright, Bondioli, Bedgak & Karl, 1983). Investigation of the relative importance of these possible sources would be facilitated by study of glucose uptake and conversion to inositol in vitro.…”

Effect of myo-inositol on the glycerophospholipid composition of adult and fetal rat lung tissue

“…The putative rate limiting enzyme in myo-inositol biosynthesis is D-glucose 6-phosphate: 1 L-my o-inositol 1 -phosphate cyclase (cyclase) [9]. In the rat, cyclase activity in primary and secondary sex organs is regulated by pituitary hormones whereas cyclase activity in the liver is regulated by thyroid hor-mones [17]. It is possible that gonadotropic studies involved short-term infusion of myohormones synthesized during pregnancy might inositol.…”

“…Possible explanations for this finding include increased myo-inositol biosynthesis in maternal tissues and decreased renal clearance of myoinositol by the maternal kidney. Increased synthesis is a likely possibility because myo-inositol synthesis in several adult rat tissues is known to be under hormonal control [17]. The putative rate limiting enzyme in myo-inositol biosynthesis is D-glucose 6-phosphate: 1 L-my o-inositol 1 -phosphate cyclase (cyclase) [9].…”