Fig. 6. Cell wall polysaccharide biogenesis via the myo-inositol oxidation pathway bold font and the sugar nucleotide oxidation pathway.
MI monophosphatase, little is known regarding its impact on availability of free MI for numer-
ous biosynthetic and regulatory requirements Fig. 1. Lithium ion delayed initiation of DNA
synthesis and cell division when introduced into synchronized Catharanthus roseus cell cultures, a
condition largely prevented when MI was in- cluded in the medium. Preparations of MI
monophosphatase from C. roseus were inhibited 80 by 10 mM Li
+
[45]. Use of Li
+
inhibition as a tool for studying modulation of free cellular
MI appears to be a viable option, one which may also alter other metabolic pathways con-
nected to a demand for free MI such as its role as substrate for MI kinase EC 2.7.1.64. Curi-
ously, little attention has been given to this latter enzyme which is present in plants, animals and
microorganisms [46]. Its product, Ins3P
1
, has the same configurational structure as that pro-
duced by Ins3P
1
synthase [5]. While one might regard this recycling of MI back into a pool of
Ins3P
1
as a salvage mechanism, it fails to take into consideration any localization of these en-
zymic activities or temporal demands during de- velopment. Together, Ins3P
1
synthase and MI kinase constitute ways in which Ins3P
1
is formed from
D
-glucose-6-P or free MI in plants. The former enzyme is biosynthetic while the lat-
ter must rely on sources that generate free MI from MI monophosphatase or other MI-conju-
gated forms. Unresolved are temporal and spa- tial patterns of synthase and kinase during
growth and development.
4. myo-Inositol oxidation pathway
Substantial experimental support for a MI oxidation
pathway MIOP
in plants
has accumulated during the past 35 years since this
pathway was first proposed [22] with over 50 papers from Loewus’s laboratory alone addressing this
topic. The presence of a MIOP has been demonstrated in a wide variety of plant tissues
including strawberry fruit, parsley leaf, lily floral parts and pollen, pear pollen, cultured sycamore
and
rice callus,
corn root-tip,
duckweed, germinating and developing wheat, pine pollen,
rubber latex serum, and algae. The MIOP Fig. 6 involves cyclization of
D
-glucose-6-P to Ins3P
1
, loss of phosphate to form MI, oxidation of MI to
D
-glucuronic acid, phosphorylation at carbon 1 a configuration, and
conversion by uridylyl transferase to UDP-
D
- glucuronic acid. Alternatively,
D
-Glucose-6-P is converted to UDP-
D
-glucose, which undergoes oxidation to UDP-
D
-glucuronic acid, a process termed the sugar nucleotide oxidation pathway
SNOP [1,2,6,21]. In Fig. 6, bold borders contrast steps of the MIOP and its metabolic products from
those of the SNOP.
Both UDP-
D
-glucuronic acid and its product of decarboxylation, UDP-
D
-xylose, strongly inhibit NAD
+
-dependent UDP-
D
-glucose dehydrogenase [47]. The requirement for NAD
+
as well as kinetic restraints imposed by product inhibition are
important considerations when invoking the SNOP for UDP-
D
-glucuronic acid metabolism. Neither of these effects appears in the MIOP. This has
significant implications in that inhibition of UDP-
D
-glucose oxidation will leave the MIOP as the principal pathway to UDP-
D
-glucuronate and its products. Relative fluxes in demands on UDP-
D
- glucose coupled to the inhibitory effect of UDP-
D
- xylose on oxidation of UDP-
D
-glucose may well allow MIOP to play a major role in hexosepen-
tose metabolism, cell wall pectin and hemicellulose formation, and starch synthesis [48 – 50]. When the
hydrogen isotope effect, which occurs during Ins3P
1
synthase-catalyzed conversion of
D
-[5-
3
H]glucose-6-P to [2-
3
H]MI, was utilized to com- pare the functional role of the sugar nucleotide
and MI oxidation pathways in germinating lily pollen [51],
3
H
14
C ratios of glucosyl and galactur- onosyl residues from amyloglucosidasepectinase
hydrolysates strongly supported the view that con- version of glucose into galacturonic acid residues
of pectin used the MIOP. Recently, UDP-
D
-glu- cose dehydrogenase has been isolated and purified
from soybean nodules [52] and cloned from soy- bean cell suspension cultures [53]. The expression
pattern of the latter was studied at selected devel- opmental stages. Results suggest that this enzyme
has a key regulatory role in production of hemicel- lulosic precursors. Access to this cloned gene for
UDP-
D
-glucose dehydrogenase
together with
those for Ins3P
1
synthase and MI monophos- phatase should provide the tools needed in future
studies to dissect expression patterns of the two pathways
at critical
stages of
growth and
development.
5. Indole-3-acetic acid IAA conjugates of MI and its glycosides
