无法获取HTTP Post请求的正确C#

我正在使用Oauth 1.0进行身份验证过程。我被困在获取请求令牌的第一步。执行以下请求会引发"远程服务器返回错误：（401）未经授权。"

        Uri uri1 = new Uri("https://api.500px.com/v1/oauth/request_token");
        string consumerKey = "ABFHGHIQYW4clmuXXXXXXXBy528gl2zEEEEN";
        string consumerSecret = "BBGHGJnrJaKKjoScCFHJGJFKK6HVjjDHFJSDJDjno";
        string timeStamp = myOAuth.GenerateTimeStamp();
        string nonce = myOAuth.GenerateNonce();
        myOAuth.includeVersion = true;
        string signature = myOAuth.GenerateSignature(uri1, consumerKey, consumerSecret, "", "", "POST", timeStamp, nonce, OAuth.OAuthBase.SignatureTypes.HMACSHA1, out normalizedUrl, out normalizedRequestParameters);
        request = (HttpWebRequest)WebRequest.Create("https://api.500px.com/v1/oauth/request_token");           
        request.Method = "POST";
        request.KeepAlive = true;
        request.UseDefaultCredentials = true;
        request.Credentials = CredentialCache.DefaultCredentials; 
        request.Headers.Add("Authorization", "OAuth oauth_callback='"http://www.abcxyz.com'",oauth_consumer_key='"" +consumerKey + "'",oauth_nonce='"" +nonce + "'",oauth_signature_method='"HMAC-SHA1'",oauth_timestamp='"" + timeStamp + "'", oauth_version='"1.0'",oauth_signature=" + signature);
        response = (HttpWebResponse)request.GetResponse();

所有参数都通过HTTP Post的"Authorization Header"传递。我的请求中肯定遗漏了一些东西，但无法弄清楚到底遗漏了什么。

任何建议或任何形式的帮助都将不胜感激。提前感谢。