问题 单项选择题

酪氨酸在体内不能转变生成的是

A.肾上腺素

B.黑色素

C.延胡索酸

D.丙氨酸

E.乙酰乙酸

答案

参考答案:D

解析:①儿茶酚胺与黑色素的合成:酪氨酸经酪氨酸羟化酶作用,生成3,4-二羟苯丙氨酸(3,4-dihydroxyphenylalanine,dopa,多巴)。与苯丙氨酸羟化酶相似,此酶也是以四氢生物蝶呤为辅酶的加单氧酶。通过多巴脱羧酶的作用,多巴转变成多巴胺。多巴胺是脑中的一种神经递质,帕金森病(Parkinson’s disease)患者,多巴胺生成减少。在肾上腺髓质中,多巴胺侧链的β碳原子可再被羟化,生成去甲肾上腺素(norepinephrine),后者经N-甲基转移酶催化,由活性甲硫氨酸提供甲基,转变成肾上腺素(epinephrine)。多巴胺、去甲肾上腺素、肾上腺素统称为儿茶酚胺,即含邻苯二酚的胺类。酪氨酸羟化酶是儿茶酚胺合成的限速酶,受终产物的反馈调节。酪氨酸代谢的另一条途径是合成黑色素(melanin)。在黑色素细胞中酪氨酸酶(tyrosinase)的催化下,酪氨酸羟化生成多巴,后者经氧化、脱羧等反应转变成吲哚-5,6-醌。黑色素即是吲哚醌的聚合物。人体缺乏酪氨酸酶,黑色素合成障碍,皮肤、毛发等发白,称为白化病(albinism)。②酪氨酸还可在酪氨酸转氨酶的催化下,生成对羟苯丙酮酸,后者经尿黑酸等中间产物进一步转变成延胡索酸和乙酰乙酸,二者分别参与糖和脂肪酸代谢。因此,苯丙氨酸和酪氨酸是生糖兼生酮氨基酸。代谢尿黑酸的酶缺陷可导致尿黑酸症。③苯酮酸尿症:正常情况下苯丙氨酸代谢的主要途径是转变成酪氨酸。当苯丙氨酸羟化酶先天性缺乏时,苯丙氨酸不能正常地转变成酪氨酸,体内的苯丙氨酸蓄积,并可经转氨基作用生成苯丙酮酸,后者进一步转变成苯乙酸等衍生物。此时,尿中出现大量苯丙酮酸等代谢产物,称为苯酮酸尿症(phenylketonuria,PKU)。苯丙酮酸的堆积对中枢神经系统有毒性,故患儿的智力发育障碍。对此种患儿的治疗原则是:早期发现,并适当控制膳食中的苯丙氨酸含量。

阅读理解

C

Our brains work in complex and strange ways.There are some people who can calculate the day of the week for any given date in 40,000 years, but who cannot add two plus two.Others can perform complex classical piano pieces after hearing them once, but they cannot read or write.

Dr.J.Langdon Down first described this condition in 1887.He called these people idiot savants.An idiot savant is a person who has significant mental impairment (损伤) , such as in autism or retardation.At the same time, the person also exhibits some extraordinary skills, which are unusual for most people.The skills of the savant may vary from being exceptionally gifted in music or in mathematics, or having a photographic memory.

One of the first descriptions of a human who could calculate quickly was written in 1789 by Dr.Benjamin Rush, an American doctor.His patient, Thomas Fuller, was brought to Virginia as a slave in 1724.It took Thomas only 90 seconds to work out that a man who has lived 70 years, 17 days, and 12 hours has lived 2,210,500,800 seconds.Despite this ability, he died in 1790 without ever learning to read or write.

Another idiot savant slave became famous as a pianist in the 1860s.Blind Tom had a vocabulary of only 100 words, but he played 5 ,000 musical pieces beautifully.

In the excellent movie Rain Man, made in 1988 and available on video cassette, Dustin Hoffman plays an idiot savant who amazes his brother played by Tom Cruise, with his ability to perform complex calculations very rapidly.

Today we more clearly recognize that the idiot savant is special because of brain impairment.Yet not all brain impairment leads to savant skills.Some studies have shown that people who have purposeful interruption of the left side of the brain can develop idiot savant skills.However few people wish to participate in such experiments.There are many excellent reasons for not undergoing unnecessary experimentation on one's brain.The term idiot savant is outdated and inappropriate.Virtually all savants have a high degree of intelligence and are thus not idiots.

63.What does the passage mainly talk about?

A.Idiot savants have areas of outstanding abilities.

B.Human Beings have complicated thinking process.

C.The brains of the idiot savants are partly impaired.

D.The reasons why people have wonderful skills vary.

64.Which of the following can be done by Rain Man?

A.He can play wonderful pieces of classical music.

B.He can guess out exactly the length of a man's life.

C.He can memorize the contents of the pictures fast.

D.He can count matches dropped on the floor quickly.

65.What can you infer from the passage?

A.Idiot savants have real talents for art and math.

B.Dr.Down is the first person who found idiot savants.

C.Few people wish to risk becoming savants by brain operations.

D.Intentional left brain impairments will surely lead to idiot savants.

66.Which of the following shows the structure of the passage?

1—5refer to paragraph 1—5.

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