Lactate recycling

Ihble 14-10. Amplification factor and characteristic diffusion time in the lactate recycling system as a function of the enzyme loading. 

Multienzyme electrodes can increase sensitivity from micromolar to nanomolar detection levels (53,57). In this case the substrate is converted to a detectable product by one enzyme, then that product is recycled into the initial substrate by another enzyme resulting in an amplification of the response signal. For example, using lactate oxidase and lactate dehydrogenase immobilized in poly(vinyl chloride), an amplification of 250 was obtained for the detection oflactate (61). 

Figure A8.18 A racemic mixture of a-hydroxyacids (like L, D-lactate) can be transformed via the corresponding a-ketoacid (pyruvate) to the desired L-amino acid (L-alanine) with cofactor recycling.

Choi and Won (1999) have reported a very u.seful strategy of recovering relatively nonvolatile lactic acid (e.g. from fermentation of carbohydrates) as volatile methyl lactate using a cationic ion-exchange resin as the catalyst. In another column reactor the methyl lactate is hydrolysed, using a cationic ion-exchange resin as the catalyst, to lactic acid and methanol, and the latter is recycled. 

Anaerobic ATP production. Recycle NADH by making lactate. 

Cooperation between liver and muscle recycles lactate into glucose. 

Titanium lactate complexes, 25 88 Titanium magnesium alloys, 13 626 Titanium-magnesium chloride recycle magnesium manufacturing processes, 15 337-338... 

Glucose produced by GNG from alanine or lactate may then be recycled to the tissues, including the muscle which provided the initial alanine and lactate. 

A key enzyme in the process of recycling lactate from muscle to liver is lactate dehydrogenase (LD) which catalyses the reversible interconversion of lactate and pyruvate. This important reaction is shown in Figure 7.10. 

Cooperation between liver and mnscle recycles lactate into glncose. 

Amplification of the sensitivity of substrate or co-en me recycling is especially efficient in thermometric analysis since all the reactions involved frequently contribute to increasing the overall temperature change. One case in point is the determination of lactate or pyruvate by substrate recycling using co-immobilized lactate oxidase and lactate dehydrogenase [160]. 

L-Iactate is oxidized by lactate oxidase to pyruvate, which is reduced back to lactate by LDH. The total enthalpy change for this system can be further increased by addition of catalase, which makes the overall enthalpy change as large as -225 kJ/mol, so signal increases greater than 1000-fold can be obtained as a result. Co-enzyme recycling was also used for the determinations of ATP/ADP [161] and NAD(H) [162],... 

The lactate formed by active skeletal muscles (or by erythrocytes) can be recycled it is carried in the blood to the liver, where it is converted to glucose during the recovery from strenuous muscular activity. When lactate is produced in large quantities during vigorous muscle contraction (during a sprint, for example), the acidification that results from ionization of lactic acid in muscle and blood limits the period of vigorous activity. The best-conditioned athletes can sprint at top speed for no more than a minute (Box 14-1). 

Batch fermentations result in high product concentration (120-150 kg/m3) but in low productivity (2 kg m Iff1). Dramatic improvements in productivity (20-80 kg m Iff1 in laboratory- and pilot-plant experiments) were obtained by using cell recycle via MF or immobilized cells, at the expense of lactate concentration in the effluent (usually lower than 50 kg/m3). 

Simultaneous L-lactic acid fermentation (by Rhizopus oryzae immobilized in calcium alginate beads) and separation was carried out using a three-phase fluidized-bed bioreactor as a fermenter (F), an external electrodialyzer as a separator, and a pump to recycle the fermentation broth between the bioreactor and the separator. In this way, the experimental specific lactate productivity and yield practically coincided with those obtained in the CaC03-buffered fermentation process (Xuemei et al., 1999), thus confirming the capability of the combined system to alleviate product inhibition without any addition of alkali or alkali salts. It was also shown that the adoption of ED-F for the production of inoculum reduced variability in inoculum quality, thus shortening the length of the lag phase of L-lactate production practically to zero as compared to that observed using an inoculum... 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

Growing concern over volatile organic compound and other emissions is motivating manufacturers to modify processes to capture and recycle solvents, reduce solvent use, or switch to solvents with better environmental profiles. A biobased solvent that is benefiting from this shift is ethyl lactate. 

Biotinidase activity in cerebrospinal fluid and the brain is very low. This suggests that the brain may not recycle biotin effectively and depends on biotin transported across the blood-brain barrier. Several symptomatic children who have failed to exhibit peripheral lactic acidosis or organic aciduria have had elevated lactate or organic acids in their cerebrospinal fluid. This compartmentalization of the biochemical abnormalities may explain why the neurological symptoms usually appear before other symptoms. Peripheral metabolic ketoacidosis and organic aciduria subsequently occur with prolonged metabolic compromise. 

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