18O isotope exchange experiments on phospholipase A2 determined by 13C-NMR: monomeric phosphatidylcholine and micellar phosphatidylethanolamine substrates

Experiments were carried out to determine whether the hydrolytic step or the product release step is the rate-limiting step for non-activated phospholipase A2 hydrolysis (Dennis, E.A. (1983) in The Enzymes, 3rd Edn., Vol 16 (Boyer, P., ed.), pp. 307-353, Academic Press, New York) of mixed micelles of phosphatidylethanolamine and Triton X-100 in the absence of activator phospholipids and of monomeric short chain phosphatidylcholine in the absence of an interface (Lombardo, D. et al. (1986) J. Biol. Chem. 261, 11663-11666). Phospholipase A2-catalyzed exchange of H2(18)O into 1-alkyl-2-[1(13)C]lauroyl-sn-glycero-3-phosphorylethanolamine and into 1-hexanoyl-2-[1-13C]hexanoyl-sn-glycero-3-phosphorylcholine were examined. Incorporation of 18O was detected by the effect of 18O on 13C chemical shifts in 13C-NMR. Both the substrate and products of the reactions were examined for 18O incorporation. 18O was incorporated into the fatty acid product, but no incorporation of 18O into the substrate was found. These results suggest that the hydrolytic step is not followed by a higher energy transition state and that it, or a step before it, is rate-limiting. Coupled with kinetic experiments, this strongly suggests that the hydrolytic step is the rate-limiting step. Thus, the role of micellar and membrane interfaces in phospholipase A2 reactions does not appear to be by aiding product removal from the enzyme active site.

Interfacial properties and critical micelle concentration of lysophospholipids

The critical micelle concentration (cmc) of several lysophospholipids and of a lysophospholipid analogue was determined from surface tension measurements using the maximum bubble pressure method and/or 31P NMR. The use of the maximum bubble pressure method has now been extended to micromolar concentrations of surfactant, and the experimental parameters that effect its use have been explored. Surface activity was found to vary with changes in the chain length and in the headgroup polarity of the lysophospholipid. The cmc's for 1-decanoyl-, 1-dodecanoyl-, 1-tetradecanoyl-, and 1-hexadecanoyl-sn-glycero-3-phosphocholine are 7.0, 0.70, 0.070, and 0.007 mM, respectively. The cmc's for 1-decanoyl- and 1-dodecanoyl-sn-glycero-3-phosphoethanolamine are 4.4 and 0.33 mM, respectively. The cmc for dodecylphosphocholine, a lysophospholipid analogue, was found to be 1.1 mM. The cmc's for 1-tetradecanoyl- and 1-hexadecanoyl-sn-glycero-3-phosphoglycerol were found to be 3.0 and 0.60 mM, respectively, in pure water. In 0.1 M Tris-HCl (pH = 8.0), their cmc's are 0.16 and 0.018 mM, respectively. Surface tension and adsorption density values determined at the cmc are reported for each compound. The relationship of dynamic surface tension and lipid purity is discussed. These studies provide information about the micellization and interfacial properties of several biologically important lysophospholipids.

Augmentation of growth hormone secretion in children with constitutional growth delay by short term clonidine administration: a pulse amplitude-modulated phenomenon

We evaluated the effect of chronic clonidine administration on 24-h integrated GH secretion (IC-GH) in eight children (six boys and two girls; age, 6.0-13.0 yr) with constitutional growth delay (CGD). Clonidine was given orally in a daily dose of 0.1 mg/m2 at bedtime for 6 months; 24-h secretion studies were performed before and after 2 months of treatment. Clonidine caused a significant augmentation (P less than 0.02) of mean IC-GH from 2.6 +/- 0.4 (+/- SE) to 4.6 +/- 0.6 micrograms/L. The increase in IC-GH was mainly the result of increased GH pulse amplitude, which rose from 12.3 +/- 1.3 to 18.2 +/- 2.1 micrograms/L (P less than 0.01). The mean GH pulse amplitude was significantly higher (P less than 0.02) during sleep (15.9 +/- 2.4 micrograms/L) than during the awake hours (8.4 +/- 1.5 micrograms/L) before treatment. During clonidine treatment the mean GH pulse amplitude during the awake hours (15.0 +/- 3.8 micrograms/L) was similar to that during sleep (20.3 +/- 3.1 micrograms/L). GH pulse frequency was not altered by treatment during either the awake or sleep hours. The mean insulin-like growth factor I levels after 2 (1400 +/- 300 U/L) and 6 (1760 +/- 430 U/L) months of treatment were significantly higher (P less than 0.02 and P less than 0.05, respectively) than the pretreatment value (920 +/- 240 U/L). After 2 months of clonidine treatment, growth velocity increased from 3.1 +/- 0.5 to 10.2 +/- 1.0 cm/yr (P less than 0.001), and after 6 months of treatment is was still significantly higher (7.0 +/- 0.7 cm/yr; P less than 0.02) than that before treatment. These results confirm the ability of clonidine to accelerate growth in children with CGD and indicate that clonidine is capable of increasing IC-GH levels. They also reinforce the view that many children with CGD have decreased endogenous GH secretion.

Rate-determining step in phospholipase A2 mechanism. 18O isotope exchange determined by 13C NMR

H2(18)O isotope exchange into specifically 13C-labeled substrate was used to obtain information on the rate-limiting step in the action of the phospholipase A2 from the venom of the Indian cobra (Naja naja naja). Incorporation of 18O was detected by the effect of 18O on 13C chemical shifts in 13C NMR. The enzymatic hydrolysis of a micellar phosphatidylcholine analogue of platelet-activating factor 1-alkyl-2-[1-13C]lauroyl-sn-glycero-3-phosphorylcholine proceeds by an O-acyl cleavage of the sn-2 ester bond. The reaction was examined for simultaneous 18O incorporation into the substrate. No exchange was found, suggesting that the hydrolytic step is not followed by a higher energy transition state and that it or a step before it appears to be rate-limiting. Previous experiments on phosphatidylethanolamine activation indicate that kcat is altered but that the km remains the same upon activation, suggesting that the binding steps occurring before the hydrolytic step are not affected. This strongly suggests that the hydrolytic step is in fact the rate-limiting step under these conditions. The 13C, 18O NMR technique should be generally applicable to mechanistic questions of this type.

Conformation of fatty acyl chains in alpha- and beta-phosphatidylcholine and phosphatidylethanolamine derivatives in sonicated vesicles

Mono- and dimethylated derivatives constitute important intermediates in the conversion of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) in eucaryote membranes. 1H-NMR techniques were utilized to examine the conformation of the region of the fatty acyl chains that is close to the polar group in the series of alpha-phospholipids: PE, N-methyl-PE, N,N-dimethyl-PE, and PC. The same series of polar groups, but on phospholipid containing sn-1 and/or sn-3 fatty acyl chains (beta-phospholipids) were also examined. All of the phospholipids were in the form of small sonicated vesicles which are widely utilized as membrane models. The alpha-methylene group of the sn-1 and sn-2 fatty acyl chains of the alpha-phospholipids give rise to separate signals due to the non-equivalency of these chains with respect to the glycerol phosphate backbone on all alpha-phospholipids tested. Additionally, differences in the environment of the PC molecules as well as N-methyl-PE, and N,N-dimethyl-PE, but not PE itself on the inside and outside of the vesicles are reflected in the chemical shift of the alpha-methylene protons. On the other hand, all of the beta-phospholipids (including beta-PE) were found to reflect the inside/outside packing differences in their alpha-methylene groups. The bilayer packing does not induce any nonequivalence in the chemically equivalent acyl chains. In mixed micelles with detergents, beta-phospholipids showed one alpha-CH2 signal for all phospholipids. These results are consistent with a common conformational arrangement for the fatty acyl chains in all alpha-phospholipids that have been investigated no matter what aggregated form. The conformational arrangement in the beta-phospholipids is different, but again is similar for all of the compounds tested in various aggregated forms.