Practical aspects of in situ 16O (gamma,n) 15O activation using a conventional medical accelerator for the purpose of perfusion imaging.

We report investigations into the feasibility of generating radioactive oxygen (15O, a positron emitter, with half-life 2.05 min) using a tuned Elekta SL25 accelerator, for the end purpose of imaging tumor perfusion. 15O is produced by the "gamma, neutron," (gamma,n) reaction between high-energy photons and normal oxygen (16O) in the body. As most in vivo 16O is bound in water molecules the 15O radio-marker is produced in proportion to water content in tissue. Imaging the washout of the 15O distribution using sensitive positron-emission-tomography (PET) technology can yield spatial information about blood perfusion in the tissue. The aim of this article was to determine the amount of 15O activity that could be produced by the tuned medical accelerator. A further aim was to model the activation process using Monte Carlo and to investigate ways to optimize the amount of 15O that could be generated. Increased activation was achieved by (i) tuning the beam to give higher-energy electrons incident on the target of the accelerator, (ii) increasing dose rate by removing the conventional filtration in the beam and reducing the source to object distance, and (iii) reducing low-energy photons by means of a carbon block absorber. The activity per-unit-dose produced by the tuned beam was measured by irradiating spheres of water to known doses and placing the spheres in a calibrated coincidence-counting apparatus. Peak energy of the tuned bremsstrahlung beam was estimated at 29 MeV, and generated activity up to 0.24/microCi/cc/3Gy in water. The measured amount of 15O agreed to within 10% of the prediction from the Monte-Carlo-computed spectrum, indicating reasonable ability to model the activation process. The optimal thickness of the carbon absorber was found to be about 25 cm. The insertion of a carbon absorber improved spectral quality for activation purposes but at the cost of reduced dose rate. In conclusion, the viability of generating 15O with an Elekta SL25 has been demonstrated. In conjunction with recent advances in high-sensitivity portable PET imaging devices, real potential exists for imaging in situ activated 15O washout as a surrogate measurement of macroscopic tumor perfusion.

The development of phonological awareness: effects of spoken language experience and orthography.

Phonological awareness, the ability to analyze spoken language into small sound units, has been shown to be affected by the individual's early orthographic experience (alphabetic vs. non-alphabetic). Past studies, however, have not differentiated the effect of script alphabeticity from that of spoken language experience, which covaries strongly with the phonological properties of the language. The present study compares younger, pre-reading to older, literate children from different linguistic backgrounds on their phonological awareness. Hong Kong and Guangzhou subjects both spoke Cantonese. The latter subjects had early experience with Pinyin (alphabetic) in addition to their logographic Chinese reading; the former read only logographic Chinese. New Zealand subjects spoke English and read the Roman alphabet. Results showed that: (1) the Hong Kong and Guangzhou pre-readers performed very similarly at all levels of phonological awareness; (2) the New Zealand pre-readers outperformed their Hong Kong and Guangzhou counterparts on onset, rime, and coda analyses; (3) the Guangzhou reading children outperformed their Hong Kong counterparts on onset and coda analyses. Whereas finding (3) reflects an effect of alphabeticity in the first learned script, finding (2) in combination with finding (1) indicates an effect of early spoken language experience independent of orthography. The fact that orthographic and spoken language experience both impact on the development of phonological skills implies a mediating function of phonological awareness in integrating sound information derived from reading and perceiving speech.

Regulation of malic-acid metabolism in Crassulacean-acid-metabolism plants in the dark and light: In-vivo evidence from (13)C-labeling patterns after (13)CO 2 fixation.

The labeling patterns in malic acid from dark (13)CO2 fixation in seven species of succulent plants with Crassulacean acid metabolism were analysed by gas chromatography-mass spectrometry and (13)C-nuclear magnetic resonance spectrometry. Only singly labeled malic-acid molecules were detected and on the average, after 12-14 h dark (13)CO2 fixation the ratio of [4-(13)C] to [1-(13)C] label was 2:1. However the 4-C carboxyl contained from 72 to 50% of the label depending on species and temperature. The (13)C enrichment of malate and fumarate was similar. These data confirm those of W. Cockburn and A. McAuley (1975, Plant Physiol. 55, 87-89) and indicate fumarase randomization is responsible for movement of label to 1-C malic acid following carboxylation of phosphoenolpyruvate. The extent of randomization may depend on time and on the balance of malic-acid fluxes between mitochondria and vacuoles. The ratio of labeling in 4-C to 1-C of malic acid which accumulated following (13)CO2 fixation in the dark did not change during deacidification in the light and no doubly-labeled molecules of malic acid were detected. These results indicate that further fumarase randomization does not occur in the light, and futile cycling of decarboxylation products of [(13)C] malic acid ((13)CO2 or [1-(13)C]pyruvate) through phosphoenolpyruvate carboxylase does not occur, presumably because malic acid inhibits this enzyme in the light in vivo. Short-term exposure to (13)CO2 in the light after deacidification leads to the synthesis of singly and multiply labeled malic acid in these species, as observed by E.W. Ritz et al. (1986, Planta 167, 284-291). In the shortest times, only singly-labeled [4-(13)C]malate was detected but this may be a consequence of the higher intensity and better detection statistics of this ion cluster during mass spectrometry. We conclude that both phosphoenolpyruvate carboxylase (EC 4.1.1.32) and ribulose-1,5-biphosphate carboxylase (EC 4.1.1.39) are active at this time.

Cytokinin Biochemistry in Relation to Leaf Senescence : II. The Metabolism of 6-Benzylaminopurine in Soybean Leaves and the Inhibition of Its Conjugation.

The metabolism of [(3)H]6-benzylamino purine was studied in presenescent and early senescent soybean (Glycine max [L.] Merr.) leaves. In both types of leaves, the metabolism was essentially the same. The principal metabolite was identified as beta-(6-benzylaminopurin-9-yl)alanine by mass spectral studies, which included discharge ionization-secondary ion mass spectrometry and pulsed positive ion-negative ion-chemical ionization mass spectrometry. Conversion to this alanine conjugate was found to be inhibited 2,4-dichlorophenoxyacetic acid and 5,7-dichloroindoleacetic acid.

Mass spectrometry and chromatography of t-butyldimethylsilyl derivatives of cytokinin bases.

Di-(t-butyldimethylsilyl) derivatives of the cytokinin bases zeatin, cis-zeatin, and dihydrozeatin may be prepared quantitatively in the presence of dimethylaminopyridine. These derivatives have good gas chromatographic properties and are very suitable for gas chromatography-mass spectrometry analysis of cytokinin bases. The t-butyldimethylsilyl (tBuDMS) group at N-9 may be selectively hydrolyzed and the resulting mono-O-silyl derivatives are sufficiently stable to be subjected to thin-layer chromatography and high-performance liquid chromatography. The mass spectral fragmentation of the mono- and di-tBuDMS derivatives of adenine, zeatin, cis-zeatin, and dihydrozeatin and also of the mono-tBuDMS derivatives of N6-isopentenyladenine and 6-benzylaminopurine have been rationalized. The 9-tBuDMS moiety was characterized by an elimination of isobutene (M-56) and of isobutene plus a methyl radical (M-56-15).

Identification of Cytokinins from Xylem Exudate of Phaseolus vulgaris L.

The principal biologically active cytokinins in xylem exudate of young Phaseolus vulgaris L. plants were identified by bioassay, high-performance liquid chromatography, enzymic degradation and combined gas chromatography-mass spectrometry (selected ion monitoring) a zeatin riboside, zeatin nucleotide, dihydrozeatin riboside, dihydrozeatin nucleotide, O-glucosyl zeatin, O-glycosyl dihydrozeatin, O-glucosyl dihydrozeatin riboside, and O-glucosyl dihydrozeatin nucleotide. Trace amounts of O-glucosyl zeatin riboside and O-glucosyl zeatin nucleotide were also detected.