Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

544 Part 5: Fruits, Vegetables, and Cereals

Sedoheptulose-7-phosphare

Transketolase

Xylulose-5-phosphate Ribose-5-phosphate

Ribulose-5-phosphate

6-Phosphogluconate

Glucose-6-phosphate dehydrogenase

Antioxidant (enzyme) system

Mitochondria

chloroplast

Membrane

degradation

SOD

POX

CAT

APX

MDHAR

GR DHAR

6-phosphogluconate

dehydrogenase

Glyceraldehyde-3-phosphate

Oxidative pentose phosphate pathway

Pyruvate

Isoprenoids

(carotenoids)

Nucleic

acid

Glucose-6-phosphate

CO 2

NADPH

NADPH

NADP+

2 H+ H 2 O

O 2 O 2 - H 2 O 2 H 2 O

NADP+NADPH

NADPH GSSG

GSH DHA

ASA MDHA

NADPH

pool

Glycolysis

NADP

NADP

Pentose phosphate

isomerase

Epimerase

Transaldolase

Erythrose-4-phosphate

Fructose-6-phosphate

Phenyl propanoid

pathway

Chalcone

Anthocyanins

Figure 27.4.Oxidative pentose phosphate pathway in plants. NADPH generated from the pentose phosphate pathway is channeled into the

antioxidant enzyme system, where the regeneration of oxidised intermediates requires NADPH. GSH, reduced glutathione; GSSG, oxidised

glutathione; ASA, reduced ascorbate; MDHA, monodehydroascorbate; DHA, dehydroascorbate; GR, glutathione reductase; DHAR,

dehydroascorbate reductase; MDHAR, monodehydroascorbate reductase; SOD, superoxide dismutase; CAT, catalase; POX, peroxidase;

APX, ascorbate peroxidase.

A peroxidase uses the oxidation of a substrate molecule (usually

having a phenol structure, C–OH, which becomes a quinone,

C=O, after the reaction) to react with hydrogen peroxide, con-

verting it to water. Hydrogen peroxide can also be acted upon

by ascorbate peroxidase, which uses ascorbate as the hydro-

gen donor for the reaction, resulting in water formation. The

oxidised ascorbate is regenerated by the action of a series of

enzymes (Fig. 27.4). These include monodehydroascorbate re-

ductase (MDHAR) and dehydroascorbate reductase (DHAR).

Dehydroascorbate is reduced to ascorbate using reduced glu-

tathione (GSH) as a substrate, which itself gets oxidised (GSSG)

during this reaction. The oxidised GSH is reduced back to GSH

by the activity of GSH reductase using NADPH. Antioxidant

enzymes exist as several functional isozymes with differing ac-

tivities and kinetic properties in the same tissue. These enzymes

are also compartmentalised in chloroplast, mitochondria and cy-

toplasm. The functioning of the antioxidant enzyme system is

crucial to the maintenance of fruit quality through preserving

cellular structure and function (Meir and Bramlage 1988, Ahn

et al. 2002).

Lipid Metabolism

Among fruits, avocado and olive are the only fruits that sig-

nificantly store reserves in the form of lipid triglycerides. In

avocado, triglycerides form the major part of the neutral lipid