1
Purple sweet potato colour – a potential therapy for galactosemia?
2
3
David J Timson*
4
5
Institute for Global Food Security, School of Biological Sciences, Queen's University Belfast,
6
Medical Biology Centre, 97 Lisburn Road, Belfast, BT9 7BL. UK.
7
d.timson@qub.ac.uk
8
*Author to whom correspondence should be addressed.
9
1
10
11
Abstract
12
Galactosemia is an inherited metabolic disease in which galactose is not properly
13
metabolised. There are various theories to explain the molecular pathology, and recent
14
experimental evidence strongly suggests that oxidative stress plays a key role. High
15
galactose diets are damaging to experimental animals and oxidative stress also plays a role in
16
this toxicity which can be alleviated by purple sweet potato colour. This plant extract is rich
17
in acetylated anthocyanins which have been shown to quench free radical production. Thus it
18
is suggested that purple sweet potato colour, or compounds therefrom, may be a viable basis
19
for a novel therapy for galactosemia.
20
Keywords: galactosemia; galactose 1-phosphate uridylyltransferase deficiency; oxidative
21
stress; anthocyanin; inherited metabolic disease; sweet potato
22
Abbreviations:
23
PSPC: Purple sweet potato colour
24
2
25
26
Galactosemia is an inherited metabolic disease affecting one in 30-60,000 newborns. It is
27
caused by mutations in the genes encoding the enzymes of the Leloir pathway of galactose
28
metabolism (Fridovich-Keil 2006). The most common form is type I, or classical,
29
galactosemia (OMIM #230400). Over 200 mutations in the gene encoding galactose 1-
30
phosphate uridylyltransferase have been associated with this disease (Bosch 2006; McCorvie
31
and Timson 2011). Types II and III are rarer and are caused by mutations in the galactokinase
32
(OMIM #230200) and UDP-galactose 4’-epimerase (OMIM #230350) genes respectively
33
(Bosch et al. 2002; Timson 2006). The manifestations of the disease vary considerably. The
34
mildest forms have altered blood chemistry and no other detectable pathology. The majority
35
of sufferers will have cataracts in childhood, which can be rectified by surgery. In more
36
severe forms, progressive and irreversible damage to the brain, liver and kidneys occurs
37
through childhood. This damage is associated with ill health and cognitive disability (Bosch
38
et al. 2002; Bosch 2006; Fridovich-Keil 2006).
39
40
There is no cure for galactosemia and current therapies rely entirely on diet. To reduce the
41
impact of organ damage, galactose and lactose (a disaccharide of galactose and glucose) are
42
largely removed from the diet (Bosch 2006; Gleason et al. 2000). Critically this means
43
eliminating dairy products and some confectionary. While these diets help minimise the
44
pathology, compliance is often an issue and there appears to be endogenous galactose
45
synthesis in the body which cannot be eliminated.
46
3
47
The molecular causes of pathology are unclear. Although large numbers of disease-
48
associated mutations have been documented there are some common, underlying molecular
49
mechanisms of their impairment. In many cases, the mutation results in a less stable variant
50
of the protein which fails to fold correctly, is more susceptible to proteolysis or forms
51
aggregates (Bang et al. 2009; McCorvie and Timson 2011; McCorvie et al. 2012; McCorvie
52
et al. 2013). This leads to reduced catalytic efficiency and thus reduced throughput in the
53
Leloir pathway. For many years, the build-up of galactose 1-phosphate was believed to be
54
the principal cause of toxicity although the mechanisms of this remain unclear (Bosch et al.
55
2002; Slepak et al. 2005; Tsakiris et al. 2002). The Leloir pathway enzymes are not only
56
required for the catabolism of galactose but are also involved the biosynthesis of UDP-
57
galactose, UDP-glucose, UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine which
58
are precursors for the synthesis of glycolipids and glycoproteins. Therefore, it has been
59
postulated that improper glycoprotein and glycolipid synthesis may also be implicated in
60
pathology and abnormal glycosylation patterns have been observed in some patients (Liu et
61
al. 2012). More recent work has established that, in a Drosophila melanogaster model for
62
type I galactosemia, oxidative stress resulting from excessive free radical production is a
63
consequence of the build-up of unmetabolised galactose (Jumbo-Lucioni et al. 2013a; Jumbo-
64
Lucioni et al. 2013b).
65
66
This partial understanding of the molecular pathology has resulted in some suggestions for
67
improved therapies. Reducing the accumulation of galactose 1-phopshate could be achieved
68
through the inhibition of galactokinase, and some highly selective inhibitors of the human
69
enzyme have been identified (Tang et al. 2012). The observation that many of the disease-
70
associated variant proteins are unstable compared to the wild-type has led to the suggestion
71
that “pharmacological chaperones” (small molecules which stabilise the affected enzyme)
4
72
could be developed (McCorvie and Timson 2013; McCorvie et al. 2013). However, no such
73
molecules have yet been identified. Free radical scavengers which mimic the action of
74
superoxide dismutase have been shown to alleviate the long-term effects of galactose toxicity
75
in the D. melanogaster model. These manganese-containing porphyrin compounds showed
76
little toxicity and thus have considerable potential as lead compounds for therapeutics
77
(Jumbo-Lucioni et al. 2013b).
78
79
It has been shown that naturally occurring plant compounds such as those in purple sweet
80
potato colour (PSPC) mitigate the effects of galactose toxicity in cells (Wu et al. 2008). This
81
appears to be due to regulation of the consequences of oxidative stress. Rats fed on a high D-
82
galactose diet had increased levels of lipid peroxidation in their liver cells; 100 mg PSPC per
83
kg body mass per day reduced this to the same level as seen in the control animals (Zhang et
84
al. 2009). This treatment also increased the activity of protective enzymes such as superoxide
85
dismutase, catalase and glutathione peroxidase (Zhang et al. 2009). The expression of pro-
86
inflammatory molecules such as NF-κB, cyclooxygenase and inducible nitric oxide synthase
87
was reduced (Zhang et al. 2009). Effects were also seen in nerve cells, where PSPC was able
88
to mitigate and reverse damage caused by high D-galactose diets (Lu et al. 2010; Shan et al.
89
2009; Wu et al. 2008). PSPC is also able to reduce the incidence of D-galactose induced
90
apoptosis (Zhang et al. 2010).
91
92
Overall, these results support the hypothesis that high dietary galactose levels result in
93
oxidative stress which can be alleviated by compounds in PSPC. By extension they also
94
support the hypothesis that the molecular pathology of galactosemia results, in part, from
95
oxidative stress resulting from the build-up of galactose. Therefore, it is a reasonable
5
96
hypothesis that purple sweet potato extracts will have similar effects in animal models of
97
galactosemia as the superoxide dismutase mimics. If so, it would be desirable to identify the
98
active ingredients of the extract and to undertake medicinal chemistry efforts to improve their
99
functionality as in vivo free radical quenchers. The main components of PSPC are acetylated
100
anthocyanins and phenolic compounds (Kano et al. 2005a; Oki et al. 2002). These have been
101
shown to quench free radical production (Kano et al. 2005b; Oki et al. 2002; Philpott et al.
102
2003).
103
104
Sweet potatoes contain relatively low levels of galactose (0.01-0.02 g per 100 g fresh mass)
105
(Kotecha and Kadam 1998) and are already recommended for inclusion in some diets for
106
galactosemia patients (Gleason et al. 2000). Therefore, it may also be worth recommending
107
the inclusion of purple sweet potato in the diets of galactosemic patients. Enhancement of the
108
levels of the active compound and reduction in the galactose levels would be desirable and
109
may be possible through selective breeding programmes (Philpott et al. 2003).
110
6
111
References
112
113
114
Bang YL, Nguyen TT, Trinh TT, Kim YJ, Song J, Song YH (2009) Functional analysis of
mutations in UDP-galactose-4-epimerase (GALE) associated with galactosemia in Korean
patients using mammalian GALE-null cells. FEBS J 276:1952-1961.
115
Bosch AM (2006) Classical galactosaemia revisited. J Inherit Metab Dis 29:516-525.
116
117
118
Bosch AM, Bakker HD, van Gennip AH, van Kempen JV, Wanders RJ, Wijburg FA (2002)
Clinical features of galactokinase deficiency: a review of the literature. J Inherit Metab Dis
25:629-634.
119
120
Fridovich-Keil JL (2006) Galactosemia: the good, the bad, and the unknown. J Cell Physiol
209:701-705.
121
122
Gleason L, Rasberry M, van Calcar S (2000) Understanding Galactosemia - A Diet Guide.
Biochemical Genetics Program, University of Wisconsin-Madison, Madison.
123
124
125
Jumbo-Lucioni PP, Hopson ML, Hang D, Liang Y, Jones DP, Fridovich-Keil JL (2013a)
Oxidative stress contributes to outcome severity in a Drosophila melanogaster model of
classic galactosemia. Dis Model Mech 6:84-94.
126
127
128
129
Jumbo-Lucioni PP, Ryan EL, Hopson M, Bishop H, Weitner T, Tovmasyan A, Spasojevic I,
Batinic-Haberle I, Liang Y, Jones DP, Fridovich-Keil JL (2013b) Manganese-based
superoxide dismutase mimics modify both acute and long-term outcome severity in a
Drosophila melanogaster model of classic galactosemia. Antioxid Redox Signal In press.
130
131
132
Kano M, Takayanagi T, Harada K, Makino K, Ishikawa F (2005a) Antioxidative activity of
anthocyanins from purple sweet potato, Ipomoera batatas cultivar Ayamurasaki. Biosci
Biotechnol Biochem 69:979-988.
133
134
135
Kano M, Takayanagi T, Harada K, Makino K, Ishikawa F (2005b) Antioxidative activity of
anthocyanins from purple sweet potato, Ipomoera batatas cultivar Ayamurasaki. Biosci
Biotechnol Biochem 69:979-988.
136
137
138
Kotecha PM, Kadam SS (1998) Sweet potato. In: Salunkhe DK, Kadam SS (eds) Handbook
of Vegetable Science and Technology: Production, Compostion, Storage, and Processing,
CRC Press, New York, pp 71-98.
139
140
141
Liu Y, Xia B, Gleason TJ, Castaneda U, He M, Berry GT, Fridovich-Keil JL (2012) N- and
O-linked glycosylation of total plasma glycoproteins in galactosemia. Mol Genet Metab
106:442-454.
142
143
144
Lu J, Wu DM, Zheng YL, Hu B, Zhang ZF (2010) Purple sweet potato color alleviates dgalactose-induced brain aging in old mice by promoting survival of neurons via PI3K
pathway and inhibiting cytochrome C-mediated apoptosis. Brain Pathol 20:598-612.
145
146
147
McCorvie TJ, Gleason TJ, Fridovich-Keil JL, Timson DJ (2013) Misfolding of galactose 1phosphate uridylyltransferase can result in type I galactosemia. Biochim Biophys Acta
1832:1279-1293.
148
149
150
McCorvie TJ, Timson DJ (2013) In silico prediction of the effects of mutations in the human
UDP-galactose 4'-epimerase gene: Towards a predictive framework for type III galactosemia.
Gene 524:95-104.
7
151
152
153
McCorvie TJ, Liu Y, Frazer A, Gleason TJ, Fridovich-Keil JL, Timson DJ (2012) Altered
cofactor binding affects stability and activity of human UDP-galactose 4'-epimerase:
implications for type III galactosemia. Biochim Biophys Acta 1822:1516-1526.
154
155
McCorvie TJ, Timson DJ (2011) Structural and molecular biology of type I galactosemia:
disease-associated mutations. IUBMB Life 63:949-954.
156
157
158
Oki T, Masuda M, Furuta S, Nishiba Y, Terahara N, Suda I (2002) Involvement of
Anthocyanins and other Phenolic Compounds in Radical‐Scavenging Activity of
Purple‐Fleshed Sweet Potato Cultivars. J Food Sci 67:1752-1756.
159
160
161
Philpott M, Gould KS, Markham KR, Lewthwaite SL, Ferguson LR (2003) Enhanced
coloration reveals high antioxidant potential in new sweetpotato cultivars. J Sci Food Agric
83:1076-1082.
162
163
164
165
Shan Q, Lu J, Zheng Y, Li J, Zhou Z, Hu B, Zhang Z, Fan S, Mao Z, Wang YJ, Ma D (2009)
Purple sweet potato color ameliorates cognition deficits and attenuates oxidative damage and
inflammation in aging mouse brain induced by d-galactose. J Biomed Biotechnol
2009:564737.
166
167
Slepak T, Tang M, Addo F, Lai K (2005) Intracellular galactose-1-phosphate accumulation
leads to environmental stress response in yeast model. Mol Genet Metab 86:360-371.
168
169
Tang M, Odejinmi SI, Vankayalapati H, Wierenga KJ, Lai K (2012) Innovative therapy for
Classic Galactosemia - tale of two HTS. Mol Genet Metab 105:44-55.
170
171
Timson DJ (2006) The structural and molecular biology of type III galactosemia. IUBMB
Life 58:83-89.
172
173
Tsakiris S, Marinou K, Schulpis KH (2002) The in vitro effects of galactose and its
derivatives on rat brain Mg2+-ATPase activity. Pharmacol Toxicol 91:254-257.
174
175
176
Wu DM, Lu J, Zheng YL, Zhou Z, Shan Q, Ma DF (2008) Purple sweet potato color repairs
d-galactose-induced spatial learning and memory impairment by regulating the expression of
synaptic proteins. Neurobiol Learn Mem 90:19-27.
177
178
179
180
Zhang ZF, Lu J, Zheng YL, Hu B, Fan SH, Wu DM, Zheng ZH, Shan Q, Liu CM (2010)
Purple sweet potato color protects mouse liver against d-galactose-induced apoptosis via
inhibiting caspase-3 activation and enhancing PI3K/Akt pathway. Food Chem Toxicol
48:2500-2507.
181
182
183
Zhang ZF, Fan SH, Zheng YL, Lu J, Wu DM, Shan Q, Hu B (2009) Purple sweet potato
color attenuates oxidative stress and inflammatory response induced by d-galactose in mouse
liver. Food Chem Toxicol 47:496-501.
8