Catabolism of myo-inositol to precursors utilized for de novo glycerolipid biosynthesis

Aberrant 3H in Ehrlich mouse ascites tumor cell nucleotides after in vivo labeling with myo-[2-3H]- and L-myo-[1-3H]inositol: implications for measuring inositol phosphate signaling

After in vivo radiolabeling of Ehrlich cells for 24h with conventional myo-[2-3H]inositol we previously demonstrated an aberrant 3H-labeling of ATP that interfered in the HPLC analysis of inositol trisphosphates. This aberrant 3H-labeling was accounted for by the extensive kidney catabolism of myo-[2-3H] inositol with delivery of 3H-labeled metabolites to extrarenal tissues. As expected, the aberrant labeling of ATP is markedly reduced with the use of 3H-myo-inositol labeled at L-C1 rather than at C2, reflecting that the 3H at L-C1 disappears in the first step of the myo-inositol catabolism: the oxidative conversion to D-glucuronate. In contrast, with the 3H at C2 of myo-inositol, the 3H-C2 passes into the pentose phosphate conversions with resulting labeling of nucleotides. The extent of catabolism to 3H-labeled water, the cellular accumulation of 3H-myo-inositol, the incorporation into cellular inositol phospholipids, and the labeling pattern of cellular phosphoinositides were all found to be similar for the two labeled myo-inositol moieties. With the use of L-myo-[1-3H]inositol an aberrant 3H-labeling at about 25% remained, for which a presumptive mechanism is proposed. L-myo-[1-3H]Inositol appears nevertheless to be a preferable alternative to myo-[2-3H]inositol for tracing the intact myo-inositol molecule after in vivo labeling, with minimized interference from aberrant 3H-labeling of nucleotides.

Aberrant 3H labelling of ATP during in vivo labelling of Ehrlich mouse ascites tumour cells with [2-3H]inositol is significant in the study of isomers of InsP3 and InsP4

Neutralized perchloric acid extracts of intra-abdominally proliferating Ehrlich mouse ascites-tumour cells harvested after 24 h exposure to [2-3H]inositol were analysed by Mono Q HR5/5 anion-exchange h.p.l.c. using an ammonium formate/phosphoric acid gradient or by ambient-pressure small-column anion-exchange chromatography (Bio-Rad AG 1-X8, 200-400 mesh). The results showed that cellular ATP contained aberrant 3H label in excess of 3H in the isomers of InsP3 and InsP4. The putative ATP 3H radioactivity showed: (i) h.p.l.c. run time as the material causing the largest A254 peak traced, (ii) precise spiking with ATP and [14C]ATP and (iii) specific absorption to charcoal. Moreover, enzymic conversion of ATP into ADP changed putative ATP 3H into putative ADP 3H. In addition, aberrant 3H labelling of cellular ADP and GTP was detected, although at a lower level.

Tunicamycin does not inhibit transport of phosphatidylinositol to Xenopus rod outer segments

Tunicamycin inhibits the dolichol pathway for N-linked glycosylation of proteins, including photoreceptor opsin, and causes a buildup of tubulo-vesicular profiles in the intersegmental space between photoreceptor rod inner and outer segments associated with disruption of new disc assembly. We tested the hypothesis that a tunicamycin lesion in photoreceptors would block lipid transport into the outer segment. Adult Xenopus retinas were preincubated in dim red light with 20 micrograms ml-1 of tunicamycin for one hour followed by incubation in the light for 2-6 h with tunicamycin plus either [3H]mannose, [3H]leucine, [2-(3)H]glycerol or [3H]myo-inositol. Tunicamycin caused accumulation of tubulo-vesicular membranes in the intersegmental space and significantly reduced both [3H]leucine and [3H]mannose incorporation into the basal region of rod outer segments. However, tunicamycin had no effect on [3H]glycerol incorporation into the rod outer segment phospholipids. After 5 h incubation with [3H]glycerol, radiolabel in outer segment fractions was associated primarily with phosphatidylinositol in both control and tunicamycin treated retinas. Quantitative light microscope autoradiography of both [3H]glycerol and [3H]inositol labelled retinas showed diffuse labelling over the entire rod outer segment in both control and tunicamycin treated retinas with no accumulation of radioactivity in the basal discs of control retinas or in the tubulo-vesicular structures in the intersegmental space of tunicamycin treated retinas. Our results indicate that despite the morphological disruption and inhibition of glycoprotein transport to outer segments after tunicamycin treatment, transport of labelled phosphatidylinositol occurs normally. These data add to a growing body of evidence separating the lipid and protein transport pathways to the outer segment.

Incorporation of [3H]inositol into phosphoinositides and inositol phosphates by rabbit blastocysts

Preimplantation rabbit embryos collected at the early morula stage were cultured to blastocysts in the presence of [3H]inositol. The blastocysts were lysed, and both the aqueous and lipid portions were analysed for incorporated radioactivity. Thin-layer chromatographic separation of the lipid portion indicated that [3H]inositol was incorporated into phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate. HPLC anion-exchange chromatography indicated that [3H]inositol was incorporated into inositol phosphates, including the two second messengers, inositol 1,4,5-triphosphate and inositol 1,3,4,5-tetrakisphosphate, and also inositol monophosphate and inositol 1,4-bisphosphate. These results provide evidence that rabbit blastocysts may have an active phosphatidylinositol second messenger system, which may be responsive to intrauterine factors or intraembryonic paracrine factors.

In vivo incorporation of [2-3H]-myo-inositol into frog opsin

The in vivo incorporation of [2-3H]-myo-inositol into frog retinal rod outer segment membranes was examined. About 25% of the recovered radioactivity was found to be protein-associated. Following acid hydrolysis of this material and extraction with hexane, all the radioactivity remained in the aqueous phase, indicating that the label was not in fatty acids. Following ion exchange column chromatography of the hydrolysate, the major radioactive compound comigrated on TLC with an internal standard of [U-14C]-myo-inositol. SDS polyacrylamide gel electrophoresis of unextracted membranes indicated that the majority of the label was associated with opsin. These results indicate that [2-3H]-myo-inositol was incorporated in vivo into opsin, presumably with retention of its chemical identity.