Dihydroxyphenylalanine, DOPA, is both an amino acid and a catecholamine (Cederbaum 1987, Rose et al 1988, Waite 1991, Goldstein 1995e, Eldrup et al 1998). DOPA is, however, most often classified as a substrate in the synthesis of classical catecholamines, dopamine, noradrenaline and adrenaline. DOPA has been found in bananas (Waalkes et al 1958) and in cereals (Hoeldtke et al 1972), dietetic tuna (Williams et al 1986), bound to proteins in canned dog food (Banwart et al 1989), and in ordinary meals (Eldrup et al 1997). Amounts of DOPA in ordinary meals were 0.6-4.7 mg, which is less than 1/100-fold the amount of other amino acids but approximately 5-fold higher than the amount of DOPA endogenously produced in 24 h (Eldrup el al 1994, Eldrup et al 1997). Wheat is unlikely to be the only source of free DOPA in ordinary meals, as breakfast and open sandwiches contained an amount of DOPA inversely related to the amount of white bread. Banana, tuna or oats was not included in our ordinary meals but the exact source of DOPA in meals was not investigated (Eldrup et al 1997).
The production of DA-S in the gastrointestinal tract may be delayed with respect to meals. DA-S produced in response to a meal may even be taken up in different tissues and from there released back to the circulation. Such a process could be passive or regulated by yet unknown mechanisms and could together with DA-S production via DA from continous DOPA synthesis and spill over to plasma explain why some p-DA-S always seems to be present even in the fasting state (Kuchel et al 1979, Eldrup et al 1997, Goldstein et al 1999) and why very high levels of DA-S are present in plasma up to several days after DA infusion (Ratge et al 1991, Nakaya et al 1994).
Dopamine (DA) is an intermediate product in NA synthesis. DA was found to be an important neurotransmitter in the autonomic nervous system with its own specific effects in the kidney and gastrointestinal tract (Thorner 1975, Christensen et al 1975). Furthermore, DA is an autocrine/paracrine substance produced locally in the kidneys (Lee 1993, Goldstein 1995g). DA synthesis and actions in the kidney, however, are beyond the scope of this thesis and will not be discussed any further. Snider & Kuchel (1983) reported evidence that free DA is derived from peripheral noradrenergic nerves and from adrenal medulla. Noradrenergic neurons may co-release DA during extreme sympathetic activation (reviewed in Goldstein 1995g). Most investigators did not observe parallel changes of p-DA and p-NA during less extreme changes of sympathoadrenomedullary activity (Bell 1988 (review), Eldrup et al 1988, Hartling et al 1989, Sothmann et al 1990). Furthermore, human antecubital venous p-DA concentrations are very low and close to the detection level of the assay (tables 1A and B, Eldrup et al 1995). P-DA concentration in man is highest in adrenal venous outflow (Snider & Kuchel 1983). Although p-DA seems to be derived from sympathoadrenal nerves, p-DA concentration is not a sensitive and useful index of sympathetic activity.
The precursor of DA, NA, and Adr, the classical catecholamines, and the immediate product of the rate-limiting step in catecholamine biosynthesis is 3, 4-dihydroxyphenylalanine (DOPA). P-DOPA concentrations in humans exceed those of NA by about 10-fold and can be measured by radioenzymatic (RE) technique or by reverse-phase high performance liquid chromatography ((rp-hplc) Zürcher & Da Prada 1979, Goldstein et al 1984). Based upon the absence of an arterio-venous increase in p-DOPA concentration in sympathectomized limbs and a decrease in p-DOPA after inhibition of tyrosine hydroxylase (TH) in dogs, it was concluded that DOPA can pass across sympathetic neuronal membranes to reach the general circulation and furthermore, that p-DOPA may be related to regional rate of tyrosine hydroxylation (Goldstein et al 1987a). P-DOPA only demonstrated minimal changes during stimuli that produced significant changes in p-NA. Due to partly parallel changes of p-NA and p-DOPA, however, it was believed that p-DOPA reflect the rate of catecholamine synthesis and that p-DOPA was a simple and direct index of TH activity in vivo (Eisenhofer et al 1988, Goldstein & Eisenhofer 1988, Garty et al 1989b). It was inferred that p-DOPA levels may be an index of sympathetic activity.
It may be that a proportionately larger increase in exocytotic NA release than NA synthesis in sympathetic nerves can explain a proportionately larger NA than DOPA response when increasing sympathetic activity (Goldstein et al 1995). No correlation between venous p-DOPA and venous p-NA concentrations has been found in contrast to the positive correlation that exists between venous p-NA concentrations and muscle sympathetic activity (Wallin et al 1981). Serial samples in the upright position would have been necessary to exclude that an increase in TH activity could increase p-DOPA levels after p-NA increase in orthostatic experiments. Results from such a protocol have never been published. Large increments in sympathetic activity have been related to small or insignificant increases in levels of p-DOPA in humans and major changes of sympathetic activity have been related to minor changes of arterial p-DOPA levels in rats and dogs. Taken together, existing evidence indicates that p-DOPA may be derived from sympathetic nerves as a result of TH activity, but p-DOPA is not a sensitive index of sympathetic activity or NA synthesis in the nerves.
From the literature there is overwhelming evidence that p-DA-S is not of sympathetic nervous origin (Kuchel et al 1985a, Ratge et al 1986b, Eldrup et al 1988, Cuche et al 1990, Eisenhofer et al 1990). Circulating free DA may be sulfoconjugated after DA infusion (Claustre et al 1990). It has been suggested by others, but not proven, that DA-S production from free p-DA takes place in the small intestine (Shibata et al 1987, Sundaram et al 1989, Eisenhofer et al 1999). P-DA-S levels increase after DDC inhibition with benserazide (Eldrup et al 1994). A continous p-DA-S production from p-DOPA via DA in the fasting state seems possible (Goldstein et al 1999) but warrant confirmation in experimental studies. The origin of p-DA-S, however, seems predominantly to be food content of DOPA, DA and DA-S (Eldrup et al 1997) though a contribution from endogenously synthesized DA in the gastrointestinal tract cannot be excluded.
In conclusion, DOPA is an amino acid and p-DOPA after meals may at least partly depend on DOPA content of meals. DOPA may be taken up from plasma and is probably released back to the circulation from depots in different tissues such as skeletal muscle and the gastrointestinal tract. Insulin may decrease p-DOPA levels but the mechanisms behind exchange of DOPA between blood and tissues have not been clarified. DOPA is synthesized from tyrosine by tyrosine hydroxylase in sympathoadrenal nerves but probably also in non-neuronal tissue in the gastrointestinal tract. DOPA is normally decarboxylated to DA, but DOPA that is not decarboxylated may spill over to plasma. P-DOPA concentration, however, is not a sensitive index of sympathetic activity or NA synthesis in nerves.
Other pathways of DOPA synthesis have been demonstrated. DOPA may be synthesized from tyrosine by tyrosinase in melanogenesis (Pomerantz 1966). Catecholamine synthesis in mice seems to be mediated by tyrosinase in the absence of tyrosine hydroxylase (Rios et al 1999). Plasma DOPA concentrations, however, were similar in albino subjects and in Caucasian and black healthy subjects (Garty et al 1989a). In seven healthy females aged 20-40 years we found no seasonal variation in p-DOPA levels when measured every 3-4 months (May, August, November and March) during one year (Eldrup et al, unpublished results). Production of DOPA from m-tyrosine has been demonstrated in rats (Hollunger & Person 1974). Furthermore, administration of 3, 4-dihydroxyphenylpyruvic acid increased p-DOPA levels in rats (Lindén 1980). When either m-tyrosine or 3, 4-dihydroxyphenylpyruvic acid were administered orally to the author in mg doses in preliminary experiments, no changes of p-DOPA concentrations were observed (unpublished results). Thus, melanogenesis does not seem to contribute significantly to p-DOPA levels in humans and no other pathways than TH mediated DOPA synthesis and DOPA content in food have been demonstrated to be important for p-DOPA levels in humans.
DOPA is synthesized by TH, the rate-limiting enzyme in synthesis of DOPA and the classical catecholamines DA, NA and Adr (Nagatsu et al 1964, Udenfriend 1966, Masserano et al 1988). TH is extensively present in vertebrates but has also been demonstrated in arthropods (for refs see Laxmyr 1985, Owen & Bouquillon 1992). TH is essential for survival, at least in mice (Kobayashi et al 1995). It has been demonstrated in dogs and rats that plasma DOPA concentrations are dependent on TH activity as TH inhibition with α-methyl-paratyrosine decreases p-DOPA levels in the basal state (Goldstein et al 1987a, Kvetnansky et al 1992a). P-DOPA levels increased concurrent with supposed massive TH activation by immobilization stress and severe hypoglycemia (Kvetnansky et al 1992a, Goldstein et al 1993). P-DOPA seemed to increase in animals but not in humans provided p-NA increased 60% or more (Table 4). P-DOPA increased significantly without DOPA clearance apparently being decreased when DDC was partly inhibited by benserazide (Fig. 6, Eldrup et al 1994). Thus, DOPA that is normally decarboxylated to DA spill over to plasma after benserazide. TH is predominantly located in noradrenergic neurons and adrenal medulla but a non-neuronal location has recently been demonstrated (Masserano et al 1988, Bäck et al 1995, Mezey et al 1996, Mezey et al 1998). How much of p-DOPA that is derived from neuronally and non-neuronally TH is unknown. The synthesis of DOPA by the TH pathway probably is most abundant in neuronal tissue which includes adrenal glands. Adrenalectomy in rats, however, did not change p-DOPA concentrations (Hansell et al 1996, Eldrup & Richter 2000) indicating that adrenal glands are not a major source of p-DOPA. P-DOPA decreased or increased after food intake, probably depending on DOPA content of meals but possibly also depending on circulating insulin levels (Williams et al 1986, O'Hare et al 1989, Eldrup et al 1997, Goldstein et al 1999). Thus, noradrenergic neurons and to some extent food seem to be sources of p-DOPA.
Very few studies have elucidated the origin of csf DOPA. Cerebrospinal fluid DOPA may partly be derived from plasma (Pletscher et al 1967), but may also be derived from DOPA synthesized in CNS as indicated by csf DOPA concentrations being lower than p-DOPA concentrations (Tables 1A and 1B). It is not known if endogenous csf DOPA concentrations in humans are higher in ventricular csf than in lumbar csf. During l-DOPA infusion a major rostrocaudal gradient of csf DOPA with highest cisternal concentrations exists, at least in monkeys (Hammerstad et al 1990). It was recently claimed that csf DOPA provides an estimate of central catecholamine biosynthetic capacity (Raskind et al 1999), but no evidence for this concept has been published. Clearance of endogenous DOPA from csf has never been elucidated.