Food Biochemistry and Food Processing (2 edition)

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27 Biochemistry of Fruits 543

is converted to lactate-by-lactate dehydrogenase using NADH

as the reducing factor, and generating NAD. Accumulation

of lactate in the cytosol could cause acidification, and un-

der these low pH conditions, lactate dehydrogenase is in-

hibited. The formation of acetaldehyde by the decarboxyla-

tion of pyruvate is stimulated by the activation of pyruvate

decarboxylase under low pH conditions in the cytosol. It is

also likely that the increase in concentration of pyruvate in the

cytoplasm may stimulate pyruvate decarboxylase directly. Ac-

etaldehyde is reduced to ethanol by alcohol dehydrogenase using

NADH as the reducing power. Thus, acetaldehyde and ethanol

are common volatile components observed in the headspace of

fruits indicative of the occurrence of anaerobic respiration. Cy-

tosolic acidification is a condition that stimulates deteriorative

reactions. By removing lactate through efflux and converting

pyruvate to ethanol, cytosolic acidification can be avoided.

Anaerobic respiration plays a significant role in the respira-

tion of citrus fruits. During early stages of growth, respiratory

activity predominantly occurs in the skin tissue. Oxygen up-

take by the skin tissue was much higher than the juice vesicles

(Purvis 1985). With advancing maturity, a decline in aerobic res-

piration and an increase in anaerobic respiration was observed

in Hamlin orange skin (Bruemmer 1989). In parallel with this,

the levels of ethanol and acetaldehyde increased. As well, a de-

crease in the organic acid substrates pyruvate and oxaloacetate

was detectable in Hamlin orange juice. An increase in the activ-

ity levels of pyruvate decarboxylase, alcohol dehydrogenase and

malic enzyme was noticed in parallel with the decline in pyru-

vate and accumulation of ethanol. In apple fruits, malic acid is

converted to pyruvate by the action of NADP-malic enzyme,

and pyruvate subsequently converted to ethanol by the action of

pyruvate decarboxylase and alcohol dehydrogenase. The alco-

hol dehydrogenase in apple can use NADPH as a cofactor, and

NADP is regenerated during ethanol production, thus driving

malate utilisation. Ethanol is either released as a volatile or can

be used for the biosynthesis of ethyl esters of volatiles.

Pentose Phosphate Pathway

Oxidative PPP is a key metabolic pathway that provides re-

ducing power (NADPH) for biosynthetic reactions as well as

carbon precursors for the biosynthesis of amino acids, nucleic

acids, secondary plant products and so on. The PPP shares many

of the sugar phosphate intermediates with the glycolytic path-

way (Fig. 27.4). The PPP is characterised by the interconversion

of sugar phosphates with three (glyceraldehyde-3-phosphate),

four (erythrose-4-phosphate), five (ribulose-, ribose-, xylulose-

phosphates), six (glucose-6-phosphate, fructose-6-phosphate)

and seven (sedoheptulose-7-phosphate) carbon long chains.

The PPP involves the oxidation of glucose-6-phosphate,

and the sugar phosphate intermediates formed are recycled.

The first two reactions of PPP are oxidative reactions medi-

ated by the enzymes glucose-6-phosphate dehydrogenase and

6-phosphogluconate dehydrogenase (Fig. 27.4). In the first step,

glucose-6-phosphate is converted to 6-phosphogluconate by the

removal of two hydrogen atoms by NADP to form NADPH.

In the next step, 6-phosphogluconate, a six-carbon sugar acid

phosphate, is converted to ribulose-5-phosphate, a five-carbon

sugar phosphate. This reaction involves the removal of a car-

bon dioxide molecule along with the formation of NADPH.

Ribulose-5-phosphate undergoes several metabolic conversions

to yield fructose-6-phosphate. Fructose-6-phosphate can then be

converted back to glucose-6-phosphate by the enzyme glucose-

6-phosphate isomerase and the cycle repeated. Thus, six com-

plete turns of the cycle can result in the complete oxidation of a

glucose molecule.

Despite the differences in the reaction sequences, the gly-

colytic pathway and the PPP intermediates can interact with one

another and share common intermediates. Intermediates of both

the pathways are localised in plastids, as well as the cytoplasm,

and intermediates can be transferred across the plastid mem-

brane into the cytoplasm and back into the chloroplast. Glucose-

6-phosphate dehydrogenase is localised both in the chloroplast

and cytoplasm. Cytosolic glucose-6-phosphate dehydrogenase

activity is strongly inhibited by NADPH. Thus, the ratio of

NADP to NADPH could be the regulatory control point for

the enzyme function. The chloroplast-localised enzyme is regu-

lated differently through oxidation and reduction, and related to

the photosynthetic process. 6-Phosphogluconate dehydrogenase

exists as distinct cytosol- and plastid-localised isozymes.

The PPP is a key metabolic pathway related to biosyn-

thetic reactions, antioxidant enzyme function and general

stress tolerance of the fruits. Ribose-5-phosphate is used in

the biosynthesis of nucleic acids and erythrose-4- phosphate

is channelled into phenyl propanoid pathway leading to the

biosynthesis of the amino acids phenylalanine and tryptophan.

Phenylalanine is the metabolic starting point for the biosynthe-

sis of flavonoids and anthocyanins in fruits. Glyceraldehyde-

3-phosphate and pyruvate serve as the precursors for the iso-

prenoid pathway localised in the chloroplast. Accumulation of

sugars in fruits during ripening has been related to the function

of PPP. In mangoes, an increase in the levels of pentose sugars

observed during ripening has been related to increased activity

of PPP. Increases in glucose-6-phosphate dehydrogenase and

6-phosphogluconate dehydrogenase activities were observed

during ripening of mango.

NADPH is a key component required for the proper function-

ing of the antioxidant enzyme system (Fig. 27.4). During growth,

stress conditions, fruit ripening and senescence, free radicals are

generated within the cell. Activated forms of oxygen, such as

superoxide, hydroxyl and peroxy radicals, can attack enzymes,

proteins, nucleic acids and lipids, causing structural and func-

tional alterations of these molecules. Under most conditions,

these are deleterious changes, which are nullified by the action of

antioxidants and antioxidant enzymes. Simple antioxidants such

as ascorbate and vitamin E can scavenge the free radicals and

protect the tissue. Anthocyanins and other polyphenols may

also serve as simple antioxidants. In addition, the antioxidant

enzyme system involves the integrated function of several en-

zymes. The key antioxidant enzymes are superoxide dismutase

(SOD), catalase, ascorbate peroxidase and peroxidase. SOD con-

verts superoxide into hydrogen peroxide. Hydrogen peroxide is

immediately acted upon by catalase, generating water. Hydro-

gen peroxide can also be removed by the action of peroxidases.