The production of L-dopa is largely dependent on the addition of specific additives and minerals to the reaction mixture. The inductive effect of diatomite on the transformation of L-tyrosine to L-dopa by Yarrowia lipolytica NRRL-143 was investigated (Figure 1). The concentration of diatomite added at the start of biochemical reaction ranged from 0.5–3.0 mg/ml, along with 3.5 mg/ml L-tyrosine. A biomass concentration of 3.0 mg/ml was used as a source of intracellular enzyme tyrosinase in a 50 min reaction. The highest production of L-dopa (1.64 mg/ml produced with 2.90 mg/ml consumption of L-tyrosine) was observed with 2.0 mg/ml diatomite. L-dopa production fell while substrate consumption continued to rise, probably due to catecholase activity causing L-dopa to be used for quinone production, since ascorbic acid (which inhibits this activity) was not being replaced in the system. In some enzyme systems, disaccharides or higher molecular weight substrates have been found to be the best supporters of intracellular enzymes [25,26]. It was hypothesized that tyrosinase, a constitutive enzyme, was altered with respect to production of L-dopa in the presence of added diatomite.
The effects of delayed diatomite addition (2.0 mg/ml; 0, 5, 10, 15, 20, 25 min) into the Y. lipolytica NRRL-143 reaction were also investigated (Figure 2). Reactions were performed aerobically with 3.0 mg/ml cell biomass and 3.5 mg/ml L-tyrosine for 50 min. Production of L-dopa increased from 5 to 15 min after the addition of diatomite; a significant decrease of L-dopa (1.68–2.14 mg/ml) was noticed 20–25 min after the addition. Maximum L-dopa (2.96 mg/ml) was obtained 15 min after the addition of diatomite into the reaction mixture, with concomitant tyrosine consumption of 2.94 mg/ml, a 35% increase when compared to the control which is highly significant (p ≤ 0.05). The L-tyrosine substrate has binding affinity with diatomite, which induces tyrosinase secretion, improves its availability and ultimately leads to an increased L-dopa production rate [7,11,13,24]. In our experiment, the addition of diatomite 15 min after reaction commencement was identified as optimal, increasing production of L-dopa, substrate utilization and time of reaction. However, L-dopa production dropped (1.68 mg/ml with 3.14 mg/ml L-tyrosine consumption) when diatomite was added 25 min after the start of reaction, probably due to conversion of unstable L-dopa to dopamine, melanin and other pigmented products [10,13] after a reduced availability of the enzyme.
The consumption of L-tyrosine, however, continued to increase despite the time of diatomite addition. The tyrosinase active center is comprised of dinuclear copper, coordinated with histidine residues, chelating substances or substances that are associated with this metal (as are quinones) which are irreversible inhibitors and/or inactivators of this enzyme [12]. The addition of diatomaceous earth may remove these inhibitors and/or inactivators by active absorption. The absorption of inhibitors increased the enzyme activity of tyrosinases, β-carboxylases and tyrosine hydroxylases which was important for the catabolism of L-tyrosine to L-dopa under controlled conditions. Our data are both substantiated [25] and in contrast to previous reports [26] in which the production of L-dopa was achieved in minimal medium without additive supplementation (pH 7.0). Previous research efforts to produce L-dopa by the addition of 0.16 μg vermiculite during the reaction obtained 0.39–0.54 mg/ml of the desired product [27].
The time course of L-dopa production and L-tyrosine consumption was carried out at different incubation periods (10–60 min) using a hotplate with magnetic stirrers (Figure 3). The control gave a maximum of 0.50 mg/ml L-dopa with 1.14 mg/ml consumption of L-tyrosine. The maximum conversion rate (3.20 mg/ml L-dopa with 3.26 mg/ml tyrosine consumption) was obtained with 2.0 mg/ml diatomite added 15 min after the start of reaction, producing a 72% higher yield of L-dopa compared to the control. The L-dopa production from this time course differed significantly (p ≤ 0.05) with the results at all other incubation periods. It is clear that up to 30 min, cresolase activity predominated and, given the non-replacement of ascorbic acid, the overriding activity was catecholase, which consumed the L-tyrosine substrate without a corresponding production of L-dopa. After 40–60 min of incubation, the production of L-dopa and the consumption of L-tyrosine decreased gradually in the control and test reactions. This reduction might be because the L-dopa and residual L-tyrosine were changed into other metabolites such as dopamine, melanin and eventually melanosine. Another study [25] achieved 0.12 mg/ml of L-dopa, 90 min after the biochemical reaction. The present finding of 3.20 mg/ml L-dopa after 30 min of incubation is a major improvement. In the present study, dopamine and melanin were also produced, but their highest production was 0.014 and 0.01 mg/ml/h.
Conversion of L-tyrosine to L-dopa is an enzyme catalyzed reaction. Figure 4 shows the effect of the addition of different concentrations of drenched cell biomass (1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 mg/ml) on the production of L-dopa from L-tyrosine in the reaction mixture. The best results (3.48 mg/ml L-dopa with 3.25 mg/ml L-tyrosine consumption) were obtained using 2.5 mg/ml wet weight yeast cells, leading to 10 fold higher productivity when compared to the control (0.72 mg/ml L-dopa with 1.22 mg/ml L-tyrosine consumption). At this concentration (2.5 mg/ml), most of the added tyrosine was converted to L-dopa as indicated by the small amount of residual substrate (0.25 mg/ml), which is highly significant (p ≤ 0.05). In the present investigation, the increased cell biomass enhanced enzymatic activity (1.55 U/mg tyrosinase). However, increasing the cellular concentration beyond the optimal led to a sharp decrease in activity, probably due to increased cell concentration (proportional to enzyme concentration) and the maintenance of a constant concentration of an inhibitor of catecholase activity (ascorbic acid). This product is the substrate for the second reaction catalyzed by this enzyme (catecholase activity) which leads to the formation of quinones from L-dopa. Only an excessive amount of ascorbic acid continually replaced throughout the reaction might stop this second activity from taking place, leading to the formation of quinones that are also suicide inactivators of this enzyme. Previous research [28] pointed out that tyrosinase activity is directly related to the concentration of cells or mycelia in the reaction mixture in slightly acidic to neutral reaction conditions. Copper atoms found at the active site of tyrosinase are an essential requirement for catalytic activity. Agents such as carbon monoxide or toxins indirectly inhibit tyrosinase activity by chelating copper and abrogating its ability to bind oxygen. Previous research [8,12,13] has shown that tyrosine phenol lyase (tpl) is only synthesized under L-tyrosine-induced conditions. The addition of L-tyrosine to the medium was found unavoidable when preparing cells (the enzyme source), but severely impeded preparation of pure L-dopa [24].
A comparison of production parameters for the effect of diatomite addition on bioconversion of L-tyrosine to L-dopa is shown in Table 1. An overall 12.5 fold increase in L-dopa production (with 4.06 mg/ml proteins) was achieved at the optimal level of added diatomite when compared to the control. The optimal pH of the control reaction without added diatomite was 3.5, however, the test reaction with added diatomite was proficient at a pH range of 2.5–4.0, indicating the enzyme remained active despite the change in reaction pH. The Yp/s value (with 2.0 mg/ml diatomite added 15 min after the start of reaction) was significantly improved over the control. Maximum substrate consumption (Qs) in terms of volumetric rate was marginally different during bioconversion between the control and test reactions, indicating maximum enzyme activity at this level of diatomite addition. The increase of qs (i.e., specific substrate consumption rate) with diatomite addition was highly significant (p ≤ 0.05). In the present study, the optimal values of all kinetic parameters (Yp/s, Qs and qs) were several-fold improved over those reported from Aspergillus or Cellulomonas spp. [7,10,28].
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